Paint shop sanding the aluminum skin

[tomn]

Just curious - if one were to strip an entire plane, what type of products/stripper would one use? I assume ailerones, elevators, rudder, flaps, etc. would need to come off - correct? Or could the control surfaces be left on and the paint left on in the inner areas if cleaned etc.? Also, whats involved in the alodine process? How much work would it take etc.?

Tom

[rob47v]

Its always best to take off all flight controls and fairings, really anything that overlaps. Most shops are applying the stripper via a pressure source, its best to apply by hand so that control is maintained as to not getting it in any seams area, but keep in mind, and this is only natural that if the hand method is performed correctly it will require in most cases that the plane be scuffed to give the stripper a cut area to seep into and do its job. I've seen areas that the stripper had seeped into and was overlooked or could not be gotten cleaned, corrosion and eaten away into the metal causing extensive repair cost to the owner. Really it is a must in most cases, but I'm not fond of stripping at all. If its done correctly and the application controlled then thats alright, but in most cases its not. Tho there are success stories.              

[PTK]

If the shop adheres to the paint manufacturer's surface prepping instructions they do not call for sanding the entire aircraft to remove paint!

Some localized sanding by hand may be required only if minor dents have to be repaired. Around rivets and general cleanup of the substrate after paint remover is washed off is also done by hand. This does not interfere or remove the alclad treatment of aluminum.

Paint systems utilize a self etching primer.

Of course these systems are very expensive and some shops may cut corners.

As in anything, it's buyer beware and you get what you pay for!

[N33GG]

If the shop adheres to the paint manufacturer's surface prepping instructions they do not call for sanding the entire aircraft to remove paint!

Some localized sanding by hand may be required only if minor dents have to be repaired. Around rivets and general cleanup of the substrate after paint remover is washed off is also done by hand. This does not interfere or remove the alclad treatment of aluminum.

Paint systems utilize a self etching primer.

Of course these systems are very expensive and some shops may cut corners.

As in anything, it's buyer beware and you get what you pay for!

Of course, your post begins with a very important "If"!

I have seen shops use little or no sanding with very good results. I have seen shops that used to have great reputations turn out crap, as well as a couple of shops that still have great reputations for that matter.

I walked in on a shop one time to see a guy with a power sander absolutely sanding off the rivet heads and deep into the skin. When I asked him how long he had been doing aircraft prep for painting, he said never, and there was no supervision to be seen. This really made me feel sorry for the owner that certainly picked up the plane with a shiny new paint job and knew nothing of what had happened.

It seems paint shops just make me nervous, and probably for good reasons. After all, what they can do to an airplane and hide under a new coat of paint...ouch! And finally, if there were no"ifs" involved, I would be much more comfortable. But alas, there are "ifs", and when it comes to paint shops, they are a big deal.

[fantom]

Of course, your post begins with a very important "If"! It seems paint shops just make me nervous, and probably for good reasons...........But alas, there are "ifs", and when it comes to paint shops, they are a big deal.

Absolutely! That's why you want a shop that's been in business for a long time, that has experienced workers with little turnover, and an owner who is a painter, not an investor, or phone marketer. No easy to find, the search is an education, and the situation is always changing. I've seen paint shops recommended here that I wouldn't let paint my bike. Be wary my friends. There is no 'best' and even if there was, no one would pay the price. Do your own detailed homework and caveat emptor.

[Andy95W]

One suggestion when you go for paint, if you don't have the bladder mod: have your paint shop sand/scuff the area around the fuel tanks and NOT use stripper in that area. I'm just talking about sanding the old paint, not sanding through to the aluminum.

There may not be anything to it, but I've worked on a number of Mooneys that needed fuel tank work 1-3 years after getting painted. It has happened enough times that it seems like more than a coincidence, and the paint stripper will definitely dissolve fuel tank sealer.

[jetdriven]

We are resealing the fuel tank panels with the correct sealer (it has hardware store clear silicone right not, cant paint over that)  It does have bladders too.

 

One thing I have noticed is all these mooneys have paint chipping off the fuel tank panel screws, which are stainless. I am putting cad plated steel screws in.

[Andy95W]

Really sounds like you've done your homework. Good luck with the new paint!

[carusoam]

Byron does the deepest detailed homework. I'm looking forward to the day when he sells the nicely prepped J to acquire a roughed over O.

Best regards,

-a-

[N601RX]

I think the bladder documents says use RTV on the inspection panels.  The only downside I can think of with PRC is that if the inspection panels ever have to be removed, it will be very difficult to do without damaging the paint. Someone had put mine on with PRC in the past and we had to remove them.  We kept working a small knife around and around them until they finally came off. Unfortunately its impossible not to damage the paint in the process. It would be nice if they could spray it with the screws out and then install the stainless ones.

Screws are a PIA when painting. If painted over and you ever have to remove them, it messes up the paint.

[cujet]

Good morning! 

 

I am DOM in a high end corporate flight department. I also have a background in extremely high end automotive painting (from a past life) and I am a moderately experienced sheet metal guy. 

 

I prefer stripping an airframe with the proper chemical stripper, a stripper that is reasonably viscous, and neutralized instantly with water. That's how the big boys at Gulfstream do it. What many don't understand is the effort required to strip an airframe properly, as the stripper will find it's way into joints, under hardware, and on to your windows. Done properly, it's the way to go. Then careful prep etch/alodine. 

 

Aluminum is an interesting metal. It's very flammable, most people don't know this. However, aluminum, when exposed to the oxygen in the air instantly forms a tough, thin, oxide coating that prevents further oxidation. Hence the inert nature of aluminum in real world use. (off topic, we can talk about thermite powder, a very reactive mix of alum and iron oxide)  The aluminum alclad on sheet metal is very corrosion resistant and very thin as mentioned in the post above. Careful use of "Scotch Brite" won't remove it. When using alodine on an aircraft, it becomes clear just where the alclad remains and where it is gone, as the color is markedly different. 

 

I generally use etch and the red "Scotch Brite" to scuff the aluminum. Rinse it and rapidly treat with alodine, which also works it's magic rather rapidly. 

 

Surprisingly, you will likely see large areas of sheet metal (even on new aircraft) where Alclad is no longer there. When manufacturers drill holes, they often sand off the burrs, sand to promote adhesion and so on. They might even sand the aluminum to remove blemishes. This all becomes clear when alodine is applied. 

 

I have been treating minor corrosion on local South Florida aircraft using a Soda Blaster. It's a viable method of stripping paint and corrosion in small areas, such as around rivets. There are some companies stripping entire airframes using Blasting Soda. It's reasonably inert with a pH of around 8.2, and instantly neutralized with water, etch, mild acids and of course Alodine. Done correctly, Soda Blasting won't remove Alclad. And it does leave a good surface for paint to bond to. However, I do not know if it's a viable method for stripping an entire aircraft. I suspect the methodology is not fully worked out. 

 

Edit: It's best to make an effort to preserve the Alclad, obviously. But don't get wrapped around the axle on this subject. It's tougher than you might guess, for example, buffing and polishing aluminum with abrasive polish won't instantly remove it. That takes years of buffing...  And, if you look carefully, you can see where it's been buffed off. Sanding with 320 will penetrate the Alclad. But, again, the manufacturer may have done this in production. 

[N33GG]

cujet:

 

Very interesting post.  Thanks!

[James]

Sorry for the wordcount, but I felt this info may help when considering paint processes.

 

We use Eldorado PR5044 paint stripper as specified by PPG Aerospace. Eldorado PR-5044 is the newest generation, nondrying, peroxide-activated paint and primer remover for removal of polyurethane and epoxy paint systems. PR-5044 is environmentally compliant, contains no chlorinated solvents and is the only peroxide stripper approved by Boeing. It is approved by or conforms to: BAC 5725, DPM 5549, Boeing D6-17487, Douglas CSD #1, Embraer, WR-ALC Purchase Description, ATR.

 

A single application of the PR5044 stripper will strip two coats of paint with an additional application possibly necessary to fully strip the airframe if the plane has had previous paint jobs done without stripping to bare metal. There is no abrasive agitation needed for this stripper, it is non-corrosive, and is washed off with fresh water and an alkaline soap to neutralize any remaining active peroxide residue . The airframe is unmasked and rewashed, with close attention paid to seams and other areas where residual stripper residue may linger to ensure there is no chance of contamination of the new coatings.

 

1. After allowing the airframe to fully dry we remove all access panels begin removing the remaining paint from masked areas. Again, this is where we like to point out where the proper process diverges from the norm because of diligence in ensuring the least destructive methods possible when preparing your airframe. We remove the remaining paint from the aircraft skin using 3M Scotchbrite products ONLY. No conventional sandpaper is used for paint removal from aircraft skin after stripping so there is no chance of inadvertently reducing the thickness of structural components such as rivet heads, window openings, etc. or altering the profile of the existing aircraft skin which reduces the aesthetics in addition to reducing the service life of the skin and making it susceptible to corrosion because of Alclad abrasion. I am currently painting a Conquest that was damaged by another "paint shop" to the tune of $275,000.00. Our structural repair department had to re-skin a considerable portion of this airframe.

 

2. The next step of the process is another area where many shops using "old methods" botch the  aircraft preservation portion of the  painting process. As specified by PPG Aerospace and in accordance with current Boeing practices, we use a conversion process in preparation for primer that does not use acid etching or an alodine oxidizing agent. The old method of preparing and converting aircraft aluminum and magnesium for priming consisted of treating the aircraft skin with acid to remove contamination and then treating with Alodine. 

 

The problem with the acid process is somewhat self explanatory because you cannot absolutely ensure complete neutralization of the acid that may have crept into the seams and airframe interior cavities, thus leaving a corrosive to sit in the lap seams and every other orifice where it will react, especially when exposed to moisture from rain or even just high humidity. This exposes the airframe to numerous opportunities for future corrosion issues. And, aside from the fact that Alodine is a Hexavalent Chromium wich is a known carcinogen, it is an oxidizer. 

 

What many do not realize about Alodine is that the application of it is to start a controlled oxidation (surface corrosion) and then stop this process by sealing it with primer. The oxidation of the aircraft skin is to provide a surface condition the primer can adhere to by creating a thin layer of oxidized aluminum. The same is true of the process for magnesium. After application of the oxidizer, you have 48 hours to seal it off with primer or the oxidation process will progress too far, resulting in adhesion failure of the primer due to exfoliation of the oxidation layer, or worse, the development of corrosion under paint.

 

This often happens when an aircraft is acid etched and alodined and then allowed to set too long, awaiting primer, because of improper management of the paint shop work schedule. This is often evident when "bubbled" looking paint is seen with corrosion underneath and no damage to the paint that would explain the development of the corrosion. Or, the aircraft skin had other contamination from traditional corrosion inhibitors due to improper surface preparation.

 

We use the PreKote system of airframe skin conversion. PreKote is non-toxic, biodegradable, and NON-CORROSIVE with a pH of 10-11.5. Chromated conversion coatings (such as alodine) are highly corrosive acids (pH of 1.3–3.0). PreKote also will not cause embrittlement or induce Cadmium leaching on Cadmium-plated metals and is approved on all composites and all metals, including magnesium and titanium. 

 

After passing numerous paint system specification tests by the US Air Force, Aerospace OEM's, and USAF flight-testing, the USAF specified PreKote as the only replacement for chromated conversion coatings in Tech Order 1-1-8, the umbrella Technical Order for aircraft painting.

 

PreKote has been qualified and is being used on the fleets operated by Continental Airlines, Air Canada, Delta Airlines, Southwest Airlines, United Parcel Service (UPS), and American Eagle among others. In addition, OEM's including Dassault Aviation and Mooney Airplane Company have approved and implemented PreKote for production aircraft.

 

3. After the conversion coating, the aircraft is masked and primed using the PPG Aerospace primer appropriate for your coatings system selection. Once again, this is another critical aspect of the aircraft painting process that is often mishandled by conventional paint shops, resulting in inferior coatings performance or outright failure of the paint system. 

 

All coatings have a period of chemical activity, meaning that even after drying to the touch the coating is "cooking" at the molecular level. Primer has a window of chemical activity, typically 72 hours for our product, in which the topcoat must be applied to achieve a chemical bonding between the two coatings. On numerous coatings systems this window may be considerably less. If the topcoat is not applied during this window, the primer must be mechanically abraded in order to get mechanical adhesion of the topcoat with the primer. This causes two major issues. 

 

First, the abrasion of the primer defeats what it's designed to do; protect the aircraft skin. All coating are designed to be applied at a minimum dry film thickness. This optimal thickness provides corrosion protection and flexibility. Without maintaining this minimum thickness, the primer can and will prematurely fail. 

 

Second, it is impossible to fully abrade every little bit of surface area like tight skin seams and around rivets. This causes weak bonding areas between the primer and topcoat which will lead to delamination between coatings. This additional abrasion process also allows the possibility of contaminating the primer with skin oils, air tool lubricants, etc. Hitting this window of molecular activity ensures that EVERYWHERE the paint flows and is in contact with the primer there is a chemical bonding of the coatings. In essence, the topcoat and primer become inseparable because they have become one coating at a molecular level. 

 

Hope this sheds some light on what you "should be getting in addition to a good "looking" paint job.

[orionflt]

and how much do you charge to strip and paint a Mooney using this process?

[carusoam]

James,

Very nice description of the process.

Welcome aboard,

-a-

[James]

Thanks carusoam...

 

This process doesn't add additional costs to the job other than minor variances for product and materials. It replaces etch/alodine, but is much less corrosive/abrasive. The devil is in the details, like after the conversion process I describe above the airframe has got to be treated as a sterile substrate. We use a mesh coverall to prevent skin oils, deodorants & fabric softener residue in clothing from fouling the substrate while masking off for prime. Also, nobody is allowed to wear cologne, hair product, etc. Smokers are made to wash their hands and forearms after breaks prior to donning latex gloves and working on the plane after the conversion process

 

We are competitive with any other proper paint shop and we are a repair station with a full structural and avionics shop in addition to our paint and upholstery operation, so much of our work is post-repair/post mods paint jobs that get included in the package price.

 

For specific pricing your scheme has to be decided, but an approximate base price would be in the $14,500 to $14,800 range for an overall solid color (non-metallic) w/ basic accent stripes. This varies by what is found after strip. I've stripped a plane before, 4 place single, and found 300+ hail dents that had been previously filled. Re skinning was in order because the feather sanding on all of those dents by the previous painter decreased the skin thickness to just above minimum, resulting in an inability to just refill them. When you are doing a pre-buy, you should be looking at the interior skin surfaces for evidence of dents that have been filled and not visible on the exterior. These may be considered pre-existing for insurance purposes if they are uncovered down the road during a re-paint and you'll be out-of-pocket for the extra cost of repair or re-skin.

 

Which brings up another issue. All old bodywork should be removed to check for corrosion. This must be handled so as not to cause skin thinning. And bodywork should never be done on bare aluminum. It should be primed first, in the area of the bodywork, and then sealed prior to overall priming to ensure the skin under the filler is protected with an appropriate Zinc/chrome epoxy primer.

 

This includes all new hdw, stainless and structural, windows sealed, etc. If you're using a shop that does not remove evrything that is not riveted on, your not getting a proper service. Of course overhead makes the price vary between shops, but you should not be looking for a cheap job unless you are flipping  the plane and not worrying about warranty.

 

Minimum warranty should be a minimum of one year. PPG Aerospace offers 3 and 5 year warranty's for adhesion, gloss and color fade based on the paint system you select.  Workmanship should be at least 6 months, plenty of time for you to find issues like runs, orange peel, over spray, etc.

 

Make sure you look at your plane extensively in a hangar with good lighting. Once you get it out of the glare of the sun, a lot of detail comes out about  workmanship. Get on your knees and look at the surface from varying angles. Pay close attention to areas of intersections like wing roots. improper paint application will cause excessive paint build up in these areas by improper overlapping of wet edge. And, eddy currents or turbulence from the paint gun can cause the paint to go thin in other areas like this, causing thin film build which results in orange peel, but more importantly, a loss of flexibility where peeling and easy chipping can occur from impacts with air debris. Also, loosen a few screws and look for indentations that would indicate the plane was reassembled before the paint cured out enough. This will cause peeling around screw heads.

[M016576]

Thanks carusoam...

This process doesn't add additional costs to the job other than minor variances for product and materials. It replaces etch/alodine, but is much less corrosive/abrasive. The devil is in the details, like after the conversion process I describe above the airframe has got to be treated as a sterile substrate. We use a mesh coverall to prevent skin oils, deodorants & fabric softener residue in clothing from fouling the substrate while masking off for prime. Also, nobody is allowed to wear cologne, hair product, etc. Smokers are made to wash their hands and forearms after breaks prior to donning latex gloves and working on the plane after the conversion process

We are competitive with any other proper paint shop and we are a repair station with a full structural and avionics shop in addition to our paint and upholstery operation, so much of our work is post-repair/post mods paint jobs that get included in the package price.

For specific pricing your scheme has to be decided, but an approximate base price would be in the $14,500 to $14,800 range for an overall solid color (non-metallic) w/ basic accent stripes. This varies by what is found after strip. I've stripped a plane before, 4 place single, and found 300+ hail dents that had been previously filled. Re skinning was in order because the feather sanding on all of those dents by the previous painter decreased the skin thickness to just above minimum, resulting in an inability to just refill them. When you are doing a pre-buy, you should be looking at the interior skin surfaces for evidence of dents that have been filled and not visible on the exterior. These may be considered pre-existing for insurance purposes if they are uncovered down the road during a re-paint and you'll be out-of-pocket for the extra cost of repair or re-skin.

Which brings up another issue. All old bodywork should be removed to check for corrosion. This must be handled so as not to cause skin thinning. And bodywork should never be done on bare aluminum. It should be primed first, in the area of the bodywork, and then sealed prior to overall priming to ensure the skin under the filler is protected with an appropriate Zinc/chrome epoxy primer.

This includes all new hdw, stainless and structural, windows sealed, etc. If you're using a shop that does not remove evrything that is not riveted on, your not getting a proper service. Of course overhead makes the price vary between shops, but you should not be looking for a cheap job unless you are flipping the plane and not worrying about warranty.

Minimum warranty should be a minimum of one year. PPG Aerospace offers 3 and 5 year warranty's for adhesion, gloss and color fade based on the paint system you select. Workmanship should be at least 6 months, plenty of time for you to find issues like runs, orange peel, over spray, etc.

Make sure you look at your plane extensively in a hangar with good lighting. Once you get it out of the glare of the sun, a lot of detail comes out about workmanship. Get on your knees and look at the surface from varying angles. Pay close attention to areas of intersections like wing roots. improper paint application will cause excessive paint build up in these areas by improper overlapping of wet edge. And, eddy currents or turbulence from the paint gun can cause the paint to go thin in other areas like this, causing thin film build which results in orange peel, but more importantly, a loss of flexibility where peeling and easy chipping can occur from impacts with air debris. Also, loosen a few screws and look for indentations that would indicate the plane was reassembled before the paint cured out enough. This will cause peeling around screw heads.

Thanks for the detail and process! What did you say the name of your shop is? Where are you located? Thanks

[James]

American Aviation, Inc.

Brooksville - Tampa Bay Regional Airport (BKV)

[BigTex]

I visited a couple paint shops and found one I liked for our Mooney 201. The only thing is, after stripping, he sands the entire plane with a DA sander and 320 grit sandpaper before acid etch - alodine - primer -paint. The more I think about this the less I like it. I read the Alclad is only 5 ten-thousandths thick per side, (3% of .032) and perhaps that's why so many airplanes I see these days have 120 bubbles of corrosion on them.The Alclad is gone. They use a Scotchbrite pad on a air drill to get around the rivets. The paint job is 9 grand, he doesnt touch the landing gear. We get that powder coated next annual.

He claims the sanding doesnt matter, and that its going to cost an extra 2 grand to remove the residue left by stripping by hand with Scotchbrite. Or he will discount it 1500$ if we deliver it stripped and shiny.

Perhaps I am overthinking this, and it isnt a big deal.

So, my options, as I see it:

-let him strip, sand, and spray it as usual, 9 grand

-pay 11 grand to have them do the job, no abrasives on the airplane.

-we strip and hand prep it with scotchbrite, get it done for 7500$

-find another paint shop. (FWIW Tejas and Murmer are real close by, but 20 grand is way out of out budget for a 201 in this category.) Artcraft in CA is 12K, but its going to take 2K to get it out there and back.

What do you think?

Byron, a buddy of mine is looking for a shop to paint his 201. What shop did you decide on using?

[jetdriven]

Hawk in Tampa

[BigTex]

Thanks... If you don't mind, what was the quoted price?

[jetdriven]

He doesn't touch the landing gear, wheel wells, and the cowling will be ready to spray in epoxy primer. 9 grand. 9500 if he had to do all that.

[BigTex]

Thanks!  He just repainted his gear so this sounds like a perfect fit.

[Michael krulcik]

I work for a great company that you can trust. My boss has 40 years experience in mena Arkansas. Its called Rose aircraft.  Say Mike from the prep crew sent you

[jetdriven]

3 minutes ago, Michael krulcik said:

I work for a great company that you can trust. My boss has 40 years experience in mena Arkansas. Its called Rose aircraft.  Say Mike from the prep crew sent you

Does he use ANY abrasives on the aircraft?