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BEST PRIMER & PAINT FOR EXPOSED STEEL PARTS ?


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Any thoughts on the best primer and paint to use for exposed steel parts on the Mooneys?  Examples like the small steel components that attach the elevator to the push pull tubes?  Or the landing gear?  Back when I worked for the Navy, powder coating wasn't recommended unless the parts were internal and non-structural.  There was a preference for epoxy primers and urethanes if I remember correctly.  Would bead blasting and a marine zinc primer and topcoat be a good idea?  I'm not painting the entire plane any time soon, just the parts that need some attention.  Obviously I'll have to get my mechanic involved but I'm looking for the best option for coatings.  Thanks.  

 

Edited by DCarlton
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28 minutes ago, N201MKTurbo said:

I always used zinc chromate. It worked great! But now it is toxic in California so you cannot get it any more. I haven't found anything that works as well. Let me know what you find.

There's a zinc phosphate available at West Marine.  Bought it for my boat.  I've used that in a few cases.  Oddly, it's green like the zinc chromate used to be.  

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20 minutes ago, DCarlton said:

There's a zinc phosphate available at West Marine.  Bought it for my boat.  I've used that in a few cases.  Oddly, it's green like the zinc chromate used to be.  

Zinc chromate dries in a few minutes and I've never had any compatibility issues with whatever I sprayed over it. Zinc Phosphate seems to be an enamel so some paints will fail if sprayed over it. And zinc phosphate takes about a month to fully dry. Also the Zinc Phosphate dries pretty thick. Zinc Chromate could be sprayed super thin and it still provided protection.

I'm not a big fan of Zinc Phosphate.  

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15 minutes ago, N201MKTurbo said:

Zinc chromate dries in a few minutes and I've never had any compatibility issues with whatever I sprayed over it. Zinc Phosphate seems to be an enamel so some paints will fail if sprayed over it. And zinc phosphate takes about a month to fully dry. Also the Zinc Phosphate dries pretty thick. Zinc Chromate could be sprayed super thin and it still provided protection.

I'm not a big fan of Zinc Phosphate.  

Can you buy Zinc Chromate in AZ?  

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1 hour ago, DCarlton said:

Any thoughts on the best primer and paint to use for exposed steel parts on the Mooneys?  Examples like the small steel components that attach the elevator to the push pull tubes?  Or the landing gear?  Back when I worked for the Navy, powder coating wasn't recommended unless the parts were internal and non-structural.  There was a preference for epoxy primers and urethanes if I remember correctly.  Would bead blasting and a marine zinc primer and topcoat be a good idea?  I'm not painting the entire plane any time soon, just the parts that need some attention.  Obviously I'll have to get my mechanic involved but I'm looking for the best option for coatings.  Thanks.  

 

As previously mentioned, Zinc Phosphate is likely the only game in town.  I just repainted my gear links using zinc phosphate as a primer. The first attempt was not up to my standards. The zinc phosphate remained tacky (enough to see fingerprints) after 3 days. I decided to attempt to apply a finish coat after waiting 4 days. When I sprayed it with automotive grade acrylic lacquer, the solvent in the lacquer softened the Zinc Phosphate primer to the point of smearing.  During the second attempt I did two things differently.  I used a much lighter touch applying the Zinc Phosphate.  Both Zinc Phosphate and Zinc Chromate should be applied so thin that the substrate is visible through the primer coat.  However, it is especially important to use a light touch with Zinc Phosphate.  The other thing I did was bake the parts the day after they had been primed. My parts were hanging on wires to dry. I used a torch to gently heat them being carful to keep the heat moving as to not bubble the primer.  With heat, I was able to cook all of the tack from the Zinc Phosphate primer.  An oven or even a hair dryer could be used to get the same result. After that, the top coat adhered well enough that I was able to wet sand the final top coat. This yielded a finish indistinguishable from factory paint. I have no idea how durable it will be long term but it has been holding up well for the last several months.

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30 minutes ago, Shadrach said:

As previously mentioned, Zinc Phosphate is likely the only game in town.  I just repainted my gear links using zinc phosphate as a primer. The first attempt was not up to my standards. The zinc phosphate remained tacky (enough to see fingerprints) after 3 days. I decided to attempt to apply a finish coat after waiting 4 days. When I sprayed it with automotive grade acrylic lacquer, the solvent in the lacquer softened the Zinc Phosphate primer to the point of smearing.  During the second attempt I did two things differently.  I used a much lighter touch applying the Zinc Phosphate.  Both Zinc Phosphate and Zinc Chromate should be applied so thin that the substrate is visible through the primer coat.  However, it is especially important to use a light touch with Zinc Phosphate.  The other thing I did was bake the parts the day after they had been primed. My parts were hanging on wires to dry. I used a torch to gently heat them being carful to keep the heat moving as to not bubble the primer.  With heat, I was able to cook all of the tack from the Zinc Phosphate primer.  An oven or even a hair dryer could be used to get the same result. After that, the top coat adhered well enough that I was able to wet sand the final top coat. This yielded a finish indistinguishable from factory paint. I have no idea how durable it will be long term but it has been holding up well for the last several months.

Your suspicions happen to be quite correct :)  On the RV project, I had used zinc phosphate spray primer (from Aircraft Spruce) on steel tubing parts and extruded aluminum parts.  Better results with multiple thin sprays than one thick layer, and if you can heat it to dry it faster, all the better.   You can hit it with a hair dryer or heat gun a minute or two after spraying (just to make sure the fumes are gone!!), there is actually a video on EAA's builder's archives doing so.  I found after 24 hours with multiple thin coats, it was still a little too tacky and soft for sanding and would scuff off if scratched, but fine for riveting.  Leaving parts over a weekend, the primer was hard enough it would withstand scratching from other aluminum parts.

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2 minutes ago, jaylw314 said:

Your suspicions happen to be quite correct :)  On the RV project, I had used zinc phosphate spray primer (from Aircraft Spruce) on steel tubing parts and extruded aluminum parts.  Better results with multiple thin sprays than one thick layer, and if you can heat it to dry it faster, all the better.   You can hit it with a hair dryer or heat gun a minute or two after spraying (just to make sure the fumes are gone!!), there is actually a video on EAA's builder's archives doing so.  I found after 24 hours with multiple thin coats, it was still a little too tacky and soft for sanding and would scuff off if scratched, but fine for riveting.  Leaving parts over a weekend, the primer was hard enough it would withstand scratching from other aluminum parts.

The Tempo Zinc Phosphate Spray bomb I used just will not toughen up without heat.  I fabricated a mount for my S.I.R.S. compass out of 2024. I cut and finished the piece, hit it with Alumiprep, rinsed primed and painted. The Tempo primer product did not stick worth a dam and pealed off in clean sheets when wet sanding.  I had to start over and scuff the allclad...then etch again...then rinse...then prime...then bake...then paint...then sand and finally polish.  It looked great until the pop rivet squeezer slipped and chipped the finish I spent hours perfecting. I am glad there were no kids around the hangars that day.

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8 minutes ago, Shadrach said:

The Tempo Zinc Phosphate Spray bomb I used just will not toughen up without heat.  I fabricated a mount for my S.I.R.S. compass out of 2024. I cut and finished the piece, hit it with Alumiprep, rinsed primed and painted. The Tempo primer product did not stick worth a dam and pealed off in clean sheets when wet sanding.  I had to start over and scuff the allclad...then etch again...then rinse...then prime...then bake...then paint...then sand and finally polish.  It looked great until the pop rivet squeezer slipped and chipped the finish I spent hours perfecting. I am glad there were no kids around the hangars that day.

I suppose there could have been a formula change, because all I did was prep with some scotchbrite pads, wipe with acetone and then primed.  The aluminum extrusions I primed with a few layers have been hit with a variety of bucking bars and didn't chip.  I've have not tried painting over the primer, though.

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I painted a new belly panel a while back and used the Bonderite prep (Alodine replacement) and then Zinc Phosphate primer (Zine Chromate replacement) and then a couple coats of Dupli-Color Oxford White spray bomb, which is a reasonable match to the paint on my airplane.   It's been holding up fine.

 

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My M20J SMM specifies a wash primer over-coated with an epoxy primer. The wash primer should be applied to bare metal. For a small area, I would use SEM Self-etching primer followed by a 2K epoxy primer. Both are available in rattle cans. That should provide excellent adhesion and corrosion resistance. For small parts any kind of color coat should be fine.

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There are plenty of fine passivation type primers. The zinc primers are galvanic primers which will provide a sacrificial anode when something wants to corrode. Zinc chromate worked fantastically well for this. From now on, we will just coat the metal with plastic and hope the evil forces of corrosion will be locked out.

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49 minutes ago, PT20J said:

My M20J SMM specifies a wash primer over-coated with an epoxy primer. The wash primer should be applied to bare metal. For a small area, I would use SEM Self-etching primer followed by a 2K epoxy primer. Both are available in rattle cans. That should provide excellent adhesion and corrosion resistance. For small parts any kind of color coat should be fine.

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Is your epoxy primer a two part?  Not sure if that's what 2K means.  Do you have a recommended manufacturer and part number?  Seems like I remember using an epoxy primer for SB208 many years ago.  Thanks.

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21 minutes ago, DCarlton said:

Is your epoxy primer a two part?  Not sure if that's what 2K means.  Do you have a recommended manufacturer and part number?  Seems like I remember using an epoxy primer for SB208 many years ago.  Thanks.

2K is shorthand for "two component" -- maybe paint guys can't spell ;) So yes, it has catalyst/hardener. There are rattle cans that have a punch button on the bottom to mix the components and then you have a several hours to use it before it hardens in the can. I don't have a favorite brand but if you search 2K epoxy primer you should find several options. 

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23 minutes ago, DCarlton said:

Is your epoxy primer a two part?  Not sure if that's what 2K means.  Do you have a recommended manufacturer and part number?  Seems like I remember using an epoxy primer for SB208 many years ago.  Thanks.

I used Sherwin Williams Aerospace two part zinc chromate epoxy primer.


https://industrial.sherwin-williams.com/na/us/en/aerospace/catalog/product/products-by-industry.11543142/mil-prf-23377k-epoxy-primer.10663142.html

 

This was years ago. I am not sure if it’s still available. I still have some but it’s long expired. It was not cheap but it was great to work with aside from the respirator. I brushed it on but you’d never know from looking at the finished product.

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7 hours ago, RoundTwo said:

I’ve used this 2-part epoxy primer on exposed steel and it’s very durable.

https://www.southernpolyurethanes.com/epoxy-primer

This is the best stuff out there for exterior steel IMHO- I have used it extensively on automotive restoration projects for the last five years and It dry or wet sands well, bonds well, and lays down very nicely. It requires no sanding before the top coat if you topcoat inside the specified window (24 hrs, I believe). The only caveat is that you can’t use an acid etch primer underneath it.

 

 

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14 hours ago, PT20J said:

2K is shorthand for "two component" -- maybe paint guys can't spell ;) So yes, it has catalyst/hardener. There are rattle cans that have a punch button on the bottom to mix the components and then you have a several hours to use it before it hardens in the can. I don't have a favorite brand but if you search 2K epoxy primer you should find several options. 

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Interesting.  Didn't know you could get a two part rattle can.  Thanks.  Two part epoxy primers seem to have consensus as being the most effective for steel... and that's what the Navy used.  Still curious what the airframe paint shops use now as a primer for the AL skins.  

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I always guessed that zinc based primers might more effective on steel since they would slow corrosion on any little area that was exposed, even on the inside.  At least intuitively, I'd imagine epoxy primers would be great unless you missed a spot, then all the corrosion would happen there.  At least bare aluminum would be less vulnerable to corrosion than steel, especially if cladded.

Of course that's just guessing, I have no idea how practically effective zinc primers are in their galvanic protection.  I suspect there's a good chance it's a trivial effect in the real world, and we're just fooling ourselves into buying zinc primers :)

 

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Older Mooneys had zinc chromate on the tubular structure and a lot had corrosion issues. The factory switched to epoxy primer somewhere in the mid-1980s I think. My 1994 has epoxy primer and looks new. From places where the finish coat has chipped, it looks like the same primer was used on the landing gear. 

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