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Any reason not to prime inside and outside of skins at the same time?

GPV

Member
Hi all,

I'm wondering if it would be ok to prime the inside and outside of my skins in one go. I haven't seen anyone do this but it would help ensure the skin is primed under the rivet head. The plan would be to prime now with Stewart Systems ekopoxy, assemble the plane then sand lightly and topcoat in a few years.

Any reason why this shouldn't be done?

Cheers,

Greg
 
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That is what I am doing, part of the reason was I have an older kit that had some corrosion that needed treatment.
 
Pro and con

Some 2 pack epoxy primers must be top coated soon after spraying otherwise they get too hard for the topcoat to stick. The spec sheet for Stewarts should specify any time limits.

Another issue I can think of is the primer will pick up oil and grime over time in the shop so it will very likely all need degreasing before topcoating.

The method I am using is to prime the insides of the skin wherever there are overlaps and contact with ribs etc. On the skin outsides I initially spray a line of primer along each rivet line and assemble. Then for final painting, scuff and degrease the whole thing, prime and topcoat. This ensures edges and rivet heads get primed.

I have heard though of some builders who have primed everything right at the beginning before they start assembling, it would save a lot of time.
 
Make sure you know exactly what your final paint finish will be, or you may be paying your painter to remove all that primer. Not all paint systems are compatible with whatever primer you may be using. Search the forums. Lots of info available.
I seem to remember a similar question coming up only a couple of months back.
Cheers, DaveH
120485
 
You mean

You mean it woul add a lot of time..

I primed only the inside of the skins and about every other part. It took tons of time. After some consideration, I can say that if I build again I probably would not prime. Do you want a 75 year aircraft or a 100 year aircraft?
 
One big consideration would be if you're planning to eventually do the top coat yourself or hire it out.

I doubt that you'll find a paint shop that will warranty their work in that situation, unless they strip all that home brew primer off and go from bare metal with their process.
 
What I do

I primed the inside only while building. The most important part of priming Alclad is the "edges" including the inside of the holes. I use a smooth foam roller to prime inside surface of the skins( slightly thin the primer), and it will coat the inward protruding dimple edge. No mess, easy to do. Done correctly, its hard to tell it wasn't sprayed. When it comes to doing the final paint, I can bet your painter will want bare metal if he is looking to warranty the life of the paint...I painted mine myself, but I have painted several planes and like to start from bare metal.
 
You mean it woul add a lot of time..

I primed only the inside of the skins and about every other part. It took tons of time. After some consideration, I can say that if I build again I probably would not prime. Do you want a 75 year aircraft or a 100 year aircraft?

I would second that sentiment... it added way too much time and hassle. I'll be long gone before the plane.
 
Thanks very much for the advice everyone!

I have to admit, the more I think about it the more I lean towards not priming at all. I'm not organised enough to do big batches and it really is slowing me down. Probably better to finish more quickly and enjoy sooner rather than having it last long after I am unable to fly.
 
Alodine

If I build another, I'll look for a company that will alodine. I'll take all the blue vinyl off and have the whole thing alodined. Zero weight gain. Probably cost way too much though.
 
R&B Aircraft Painting in Topeka KS advertises that part of their process is an acid etch and allover alodine treatment before primer.

No idea what it costs since that's still several years down the road for me, but there's a picture of an RV on the "process" page of their website where they mention it.
 
Alodine process outsource.. Why?

Alodine is about as easy as it gets, but really its primary function is a conversion coating for better topcoat adhesion. The acid etch process is an alternative to mechanical etch, IE Scotchbrite or similar. Ultimately, the most susceptible parts of our planes are the extrusions not able to take advantage of Alclad which is simply stated a pure aluminum, relatively non-corrosive layer applied to the sheet alloys. Alodine can be applied via cheesecloth/cotton rag wipe on, spray or submersion. Takes only a few minutes, and a good water rinse to be completed. A large plastic bag with a few ounces in it can be used to slosh around small pieces and such. All of my skins were alodined using cheese cloth an rubber gloves..we do it every day in the big jet overhaul world. There are even Alodine marker pens, similar to a big fat dry-erase that you can apply it with and no rinse is required. The skin edges, holes, and any extruded parts are the most important area of primer application while building if you want protection for eternity..that's where the corrosion will start.
 
I have the same problem...

I have had corrosion during the build process on "not started" material. Usually it starts like a hairline at the punched wholes.

That is why I have been in the "prime everything" camp. It sucks. I hate priming. It adds years!

I may try to just shoot the outside where things are riveted and leave the rest bare. I dont think it will corrode there. and if it does there is little harm. I can always shoot it from the outside. But I will never be able to get under the rivets unless I spray before assembly.

just my 2 cents.
 
You mean it woul add a lot of time..

I primed only the inside of the skins and about every other part. It took tons of time. After some consideration, I can say that if I build again I probably would not prime. Do you want a 75 year aircraft or a 100 year aircraft?

But, at the same time, it is incredibly disheartening to find corrosion on your mint airplane a few years after getting it flying. There is a five year-old bare metal RV in a hangar near me that is COVERED with corrosion. I don't think the owner has seen it in a couple years, but in coastal areas, Alclad is not enough.

As others have said, a paint shop will strip the exterior back to bare metal, so if you are farming the final paint out, any exterior priming is for the build phase only.
 
Do you want a 75 year aircraft or a 100 year aircraft?

I've decided NOT to prime for this as well as another reason. Since I will have built it, if necessary and if Van's is still around, I can just rebuild whatever part(s) needed.
 
But, at the same time, it is incredibly disheartening to find corrosion on your mint airplane a few years after getting it flying. There is a five year-old bare metal RV in a hangar near me that is COVERED with corrosion. I don't think the owner has seen it in a couple years, but in coastal areas, Alclad is not enough.
SNIP
+1
On the first RV I primed every square in of the interior and any overlapping parts with a quality two part epoxy primer. I flew the plane for two years, the plane always hangared. I was disturbed with the rapid external surface corrosion before I got a chance to paint the exterior.

My point - corrosion cares little about internal or external aluminum. Prime the interior parts and overlaps during construction. I would however caution against priming the exterior until ready for final paint. There are time limits from prime to top coat that must be met. While many primers say you can “scuff up cured primer then top coat”, I have first hand experience that this will yield very disappointing results. If you do that, you will be forced to prime the exterior again anyway.

I also offer that a good quality epoxy primer has far superior abrasion resistance to alodine type chemical conversions.

Carl
 
There is a five year-old bare metal RV in a hangar near me that is COVERED with corrosion. I don't think the owner has seen it in a couple years, but in coastal areas, Alclad is not enough.

You have to ascertain the corrosion type before hitting the alarm bell. Aluminum will often get a surface coat of corrosion and that corrosion then acts as a barrier to prevent further corrosion. Typically this corrosion forms in a few minutes and is invisible. However, in some conditions it is a bit thicker and clearly visible. VERY different than steel or iron, where the surface corrosion helps to trap moisture and accelerates deeper corrosion. Often this corrosion is well less than .001" deep and doesn't affect structural integrity.
 
+1
I would however caution against priming the exterior until ready for final paint. There are time limits from prime to top coat that must be met. While many primers say you can “scuff up cured primer then top coat”, I have first hand experience that this will yield very disappointing results. If you do that, you will be forced to prime the exterior again anyway.

Carl

I prime inside and out, then once I assemble it and get ready to final paint the part I do sand quite a bit, do any necessary body work, then re-prime and final paint. Adds more work and a little more weight. I am also concerned with bare parts corrosion. I have rapidly moved through the build, but am planning for the worst. Never know what life brings and I might have to slow down on the build. I do live in a costal region and everything corrodes. I did recently insulate, add an A/C and dehumidifier in the hangar; this has slowed the corrosion down.
 
While many primers say you can “scuff up cured primer then top coat”, I have first hand experience that this will yield very disappointing results.
Carl

+1

All recommendations that I have seen for PU paint is that old primer must be scuffed and a new coat of primer applied before top coat for good results. You may be able to get away with it with single stage, but definately not with base /clear.

Larry
 
I would pass on any interior priming that is not going to be painted. it just isn't worth it for the homebuilder. it's effort, time, exposure to chemicals and cost for very little gain.

1. effort
2. time
3. exposure to chemicals
4. cost

= very little gain. heck, he could have been flying by now if he didn't prime the interior......
 
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