Paint shop sanding the aluminum skin

[jetdriven]

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?

[N601RX]

Did you get a chance to find out what the more expensive shops you mentioned do? I've saw a local paint shop do the same thing and didn't really think it was a good long term idea.

[DS1980]

I'd go the $7500 route, if you have the time. Then you know what condition your metal is under the paint. Worth a few sore shoulders in my book.

[N33GG]

I have seen shops sanding.

Nobody will ever sand my aircraft. Just my position on this matter.

YMMV IMHO etc

[johnggreen]

I've been trying to settle on a paint shop for my Aerostar for almost six months.  About the only sure conclusion I've reached is that these guys have been smelling way too many fumes.  In my experience, which is pretty extensive, there is no aircraft maintenance area with such a high percentage of nuts, flakes and incompetence.

 

I called one highly touted shop which demanded a 50% deposit three months before the scheduled delivery.

 

I had my Skylane painted last spring and after picking it up found a host of oversights including stop screws left off aileron pins, painted light sockets, landing light wires left unconnected, wires not secured and left to hang against exhaust, not to mention the week of paint touch up that I had to go back for.

 

As for sanding.  If I caught one sanding my aluminum, I'd sue him and I'm NOT kidding.

 

Just my opinion.

 

Jgreen

[Hector]

There is absolutely no need to remove the ALCLAD layer and I would be furious if I saw a paint shop doing that to my airplane.

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[MooneyMitch]

You might querry Teresa at ArtCraft. 

[Alan Fox]

You get what you pay for , as far as sanding , you have to scuf the metal to get the paint to adhere , even if you strip and alodine , if not the paint flakes off in ten years or so , 9K for a strip and paint is very low , Beware of prices too low...... As long as they don't sand off rivet heads it will be fine , a quick hit with a DA with 320 on it will not remove the Alclad , unless you sit there with it and hold it one spot , they are probably going to scuff the metal , the whole plane in about an hour after it is stripped and washed....  I saw a king air in an insurance auction a few years back where the paint shop sanded off the rivet heads... What a mess.....

[TWinter]

18 months later and mine is still as nice as the day I picked it up. Some shops will occasionally pick up a plane when they are delivering one completed. I think I paid and extra $300 to have a ferry pilot take my plane back to Mena, AR from Dyersburg, TN. I spent more than that paying a buddy's fuel for his Skylane  and having him take me up and back when it was done..plus I gave him gift certificate for him to go out to dinner for his time. Should have waited and let them bring it back, would have been cheaper in the long run. 

 

Not sure about the scuff deal. My shop never mentioned that part of the process. May have done it, but even reading up prior to getting it painted I don't recall much about that. The acid bath and cleaning/prep was the most important in most discussions I read. My shop did take the time to exam all rivets and replaced a few as precaution.  Just my .02

Mine was under 11K. One of the guys called Mena a few days ago about doing a different Skylane..I think he was quoted 9K.

[rob47v]

N74795 is correct as long as its acid washed re-anodized, and get all the stripping solution off, you will be fine. The re-anodizing will give the aluminium the corrosion protection it needs, followed by a good coat of zinc primer. I'm not fond of stripping  just because their is a chance of not totally removing all the stripping solvent and problems arising at a later date. If the paint or the old paint is decent should we say, not flaking nor crazing then I would use it as base. It always works and works great. I'm an A&P and at work anodizing is a must and it must be verified with every job we do! The problem with stripping is when the stripper isn't removed entirely and get btwn the seams. Well then let your imagination run wild as to what happens next.     

[takair]

Looked it up in the maintenance manual and step "a" includes sanding as needed. They suggest starting at 400 and ending at 600 or 800. Would be curious what the factory does. I think more shops, than not, sand. Regarding the alclad, the surface treatments, including alodine, primer and paint should prevent corrosion. I do know that it is prohibited on some pressurized aircraft and more than one has been scrapped as a result of sanding, but it appears in our case that it is actually part of the official process. I personally hate the thought, but......

[bumper]

N74795 is correct as long as its acid washed re-anodized, and get all the stripping solution off, you will be fine. The re-anodizing will give the aluminium the corrosion protection it needs, . . .

 

I'm betting you meant to say re-alodined rather than re-anodized. 

 

Alodine is a surface conversion process. Anodize would involve dumping the part (in this case the whole airplane) in a dilute sulfuric acid bath and attaching DC positive (anode) to it and DC negative (cathode) to the solution via a lead plate or similar! Alodine makes a thin and relatively fragile surface while anodizing makes a layer of very hard and durable aluminum oxide. 

 

Unfortunately, since anodizing creates a thin, hard, egg-shell like surface layer, if there's flexing of the part, the harder surface can crack and that crack can then propagate into the underlying parent metal - - this is why anodizing is not often used for structural components. 

 

To test a surface to see if it's anodized or not, you can use an ohm meter and touch the probes to the surface - the anodize layer is an insulator so no reading.

 

Sorry - - couldn't help myself (grin) . . . I have a small anodizing set up at my hangar machine shop, so love this stuff.

 

bumper

[mike_elliott]

I'm betting you meant to say re-alodined rather than re-anodized. 

 

Alodine is a surface conversion process. Anodize would involve dumping the part (in this case the whole airplane) in a dilute sulfuric acid bath and attaching DC positive (anode) to it and DC negative (cathode) to the solution via a lead plate or similar! Alodine makes a thin and relatively fragile surface while anodizing makes a layer of very hard and durable aluminum oxide. 

 

Unfortunately, since anodizing creates a thin, hard, egg-shell like surface layer, if there's flexing of the part, the harder surface can crack and that crack can then propagate into the underlying parent metal - - this is why anodizing is not often used for structural components. 

 

To test a surface to see if it's anodized or not, you can use an ohm meter and touch the probes to the surface - the anodize layer is an insulator so no reading.

 

Sorry - - couldn't help myself (grin) . . . I have a small anodizing set up at my hangar machine shop, so love this stuff.

 

bumper

 

[mike_elliott]

How cool! Do you cad plate your own hardware also?

[WardHolbrook]

You'd peaked my curiosity. In the past we've used Duncan Aviation to paint 7 or 8 of the company's corporate jets. Their paint jobs weren't cheap, but they lasted and looked brand-new after 7 years and around 4000 hour on each airframe. When it comes to painting, it's all about the prep work. About 3 years ago we had one of our Falcon 900s repainted and "they" decided that we would  go with another shop that gave us a bit better quote. It's becoming obvious that this one isn't going to last nearly as well. I will see if I can find out the way Duncan does it on the upscale corporate jet projects and report back. 

[Hector]

Yes, some sanding is always required, but there is absolutely no point on purposely trying to remove the ALCLAD layer to get to the base metal. Alodine will restore some corrosion protection (along with primer and paint), but alodine is not as effective as ALCLAD.

[Alan Fox]

^{Actually alodine creates a layer of corrosion , the same as blueing a gun .... just a controlled surface prep , as far as sanding , they are not sanding off the layer of alclad , they are scratching , or scuffing the surface for better adhesion of the base coat.......}

[Hector]

^{Actually alodine creates a layer of corrosion , the same as blueing a gun .... just a controlled surface prep , as far as sanding , they are not sanding off the layer of alclad , they are scratching , or scuffing the surface for better adhesion of the base coat.......}

It's a chemical conversion process and indeed creates a layer of corrosion, but inferior to ALCLAD. Nothing wrong with scuffing the surface, but no need to deliberately remove the ALCLAD. I found some guys in our paint shop (large airframe overhaul facility) doing just that because they thought it was better to replace the ALCLAD with fresh alodine. Had to gather around everyone in the shop for some re-education.

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[bumper]

How cool! Do you cad plate your own hardware also?

Would if I could. Darn near impossible to buy cadmium though. A friend has some "pure" cadmium - - uses it to plate in white the bits and pieces on Brit motorcycles he restores. Star of the show is a very nice Vincent Black Shadow. He stocks my hangar fridge with root beer and ginger beer and I help him now and then fixing little bits of broken bikes.

 

bumper

[rob47v]

Thank you dear sir for the correction, And if any scuffing is performed the alclad is destroyed. Sorry but no matter which way anyone likes to see it. At my job we use epoxy zinc chromate. Toughest stuff I've ever seen Nothig penetrates.

I'm betting you meant to say re-alodined rather than re-anodized.

Alodine is a surface conversion process. Anodize would involve dumping the part (in this case the whole airplane) in a dilute sulfuric acid bath and attaching DC positive (anode) to it and DC negative (cathode) to the solution via a lead plate or similar! Alodine makes a thin and relatively fragile surface while anodizing makes a layer of very hard and durable aluminum oxide.

Unfortunately, since anodizing creates a thin, hard, egg-shell like surface layer, if there's flexing of the part, the harder surface can crack and that crack can then propagate into the underlying parent metal - - this is why anodizing is not often used for structural components.

To test a surface to see if it's anodized or not, you can use an ohm meter and touch the probes to the surface - the anodize layer is an insulator so no reading.

Sorry - - couldn't help myself (grin) . . . I have a small anodizing set up at my hangar machine shop, so love this stuff.

bumper

[Piloto]

On some areas sanding is necessary to remove surface corrosion. otherwise the primer or paint will not bond properly. Any sanded surface must be coated as soon as practical to prevent corrosion build up.

 

José 

[rob47v]

True sanding is necessary for proper bond. Its the treatment process that must be done correctly to avoid corrosion. 

[N601RX]

I've saw localized corrosion removed with soda blasting.

[jetdriven]

We've had good luck removing localized corrosion with a scotch brite either by hand or a rolox wheel, or a nylon abrasive wire wheel in a drill. We don't have much at all.

[KSMooniac]

I'm soured on work in Mena in general after my experiences there.  I think there is an underlying cultural problem with that area about following directions, using maintenance manuals, etc.  There is no reason to sand an entire airframe IMO!  Especially with unskilled labor.  Are you even sure the Mooney skins have a clad layer under the paint?  Sheet aluminum can come in clad (both sides), clad (one side) or un-clad.  It wouldn't surprise me if only the interior side were clad since the exterior is presumably protected by paint... if that is the case then sanding will be eating into the structural aluminum.

 

If I were to do it over, I might consider your option of stripping and prepping myself/at home, and then delivering to a shop for the paint.  You don't need a clean hangar to strip and alodine... just some anal retentive tenacity.  I'm worried about my paint shop's lack of diligence and the durability of my paint.  At the next annual after paint I found corrosion in my spinner bulkhead where they didn't get the stripper cleaned sufficiently.  Hell, they should have removed the spinner prior to applying the stripper in the first place, especially since I gave them a freshly polished spinner to install!  I wonder what else they didn't do properly...