Prep & Primer thickness

[johnk]

All,

I spent many hours reading internet postings, manufacturer's data sheets, and talking to EAA members. I spent many hours thinking about the subject. I changed my mind numerous times. Finally, with parts piling up, I said "time's up, do something". At that point, my intended process had evolved to:

(1) Wipe down with acetone
(2) Part A, Wash with Dawn dishwashing detergent
(3) Part B, Wash with Dawn dishwashing detergent while lightly scuffing surface with gray scotchbrite pads
(4) Blow dry with air gun
(5) Paint with three light coats of Marhyde (or, is it Mar-hyde?)
(6) Let dry and harden for 24 hours

Before I got to this point I had considered many other options:
- Do nothing (i.e., rely on Alclad)
- MEK for cleaning
- Alumiprep and Alodine
- Tempo Zinc Chromate
- Tempo Zinc Oxide
- Epoxy Primer
- AFS
- Rustoleum Primer
- Topcoat/No Topcoat
- other...

The selection reduced to two problems I could not resolve:
(1) The chemical toxicity of all of these substances is a significant concern for me. I have two small kids in the house and don't feel comfortable with lots of industrial-strength chemicals around, in the air, the yard, etc.
(2) The main reason for applying these coatings is to prevent corrosion. Unfortunately, I could find very little info documenting their long-term performance in this area.

So, how does one perform a tradeoff between performance, hazard, and ease-of-use if performance data is not available? I don't know. So I chose the method/product that seemed easy to use, is only slightly deadly if used properly, and is on Van's list in Section 5 of the instructions.

Peter, sorry, I didn't really mean to imply that I hated the process, I meant that it was much less enjoyable for me than the metal work. I think I'm just a bad painter. Nevertheless, everything came out looking pretty nice when done, and it REALLY LOOKS GREAT RIVETED TOGETHER!! (my basement now looks like a Boeing airplane factory!)


P.S. Given the importance of corrosion prevention, and given the lack of solid performance data on the wide variety of approaches available to the amateur builder, I would like to propose that the EAA commission a technical study of this subject. I suggest they hire a suitable expert in the field and have him/her spend a year or so evaluating the complete range of candidate processes practical for the home builder. Then tell us how the various approaches compare with respect to (1) corrosion prevention performance, (2) safety/toxicity, (3) cost, and (4) ease-of-use.

[captain_john]

John, Excellent summary if I do say so myself!

That is what I call due diligence and a well formed opinion. An opinion which, I agree with... so that makes things easy!



One note, 3 coats of Mar-Hyde? Product brand is irrelevant. The coating requirements depend on manufacturer's suggestions. The "mil coating" requirements are on the documentation. From what I have seen, most primers spec out one thin coating, about 1 mil in thickness. A "tack coat" is normally applied and once it flashes a cross hatched layer is then shot. This IS NOT 2 coats. In order to qualify as 2 coats, a complete curing process needs to be accomplished. That normally takes 24 hours, a scuffing and then the second application.

Agreed?

CJ

[johnk]

I thought I was following the directions on the can. I don't have it in front of me, but it said something like: Apply multiple thin coats instead of one thick coat, wait 10 minutes between coats, wait 2 hrs for topcoat.

Are you saying that a single "coat" is actually a two-step process, first a "tack coat" and then a "cross hatched layer".

[avaviat]

Just a quick question... why acetone (or MEK) and dish soap pre-wash? I guess I'm asking more about an alodine process than a self-etch primer process.

I prepped a few parts w/ MEK+dish soap+Metal Prep #79, and others with Metal Prep alone. They all came out clean and oil free. The alodine went on identically. I couldn't tell the difference really. Metal prep even removed the "price tag" lables and blue sharpie writing, as neat as anything.

My plan is to alodine the ribs/spars/etc, and epoxy prime exposed structural parts (e.g. the aft spar on the h. stab). The goal is to build light but I'm ~50 miles to the ocean so I want some added corrosion protection.

[captain_john]

John, lemme back up and talk about curing a bit. First you shoot the coating. It is glossy and has a sheen to it. This is obviously wet and while shooting you should "drag the wet spot" along as you shoot.

After it turns dull but is still tacky and soft, this stage in curing is called "flashed off" or "after it flashes". This is the time to do the cross hatched layer. It is still technically one coat as the activator in the paint will reactivate the first "tack coat" and bond with it.

Now, some coatings take longer to fully "cure" than others. Typically it is agreed that after 24 hours all types of paints will be considered "cured". It is after this time that if a second coat is applied the surface needs to be "profiled" before another coat can be applied.

If another coat is applied before this 24 hour window is applied, it can be called a "second coat" only if the first is fully cured according to the specs on the product.

Most processes recommend a thin (about 1 mil) primer coat and a top coat within 24 hours, otherwise profiling is nesessary. Personally, if I were to re-profile, I would also re-prime, as this will allow the coatings to bond more solidly.

Now, I aren't a en-ga-near but... I have learned all this from a good friend who is an en-ga-near and also enjoys Captain Morgan's Rum. We dwell in the minutia of coatings and discuss women until the rum is gone.

Soooooo, if this helps!?!

Do you have any Captain in ya?

CJ

[captain_john]

Oh... and about metal prep.

A solvent clean is recommended as per ASTM standards. That is exactly what I do. Dishwashing liquid is a solvent last I checked and the scotchbrite pad offers a GREAT profile!

A wipe with MEK, Laquer Thinner, Acetone or just about anything that leaved a clean oil free surface is good in my book. This probably isn't necessary, but I often do it to remove any silicones or other stuff that may or may not be in the dishwashing liquid.

CJ

[johnk]

CJ,

Thanks for all the info. Sooner or later I'll get the hang of priming (probably later). I'm still not sure how to figure out how thick the paint is, but, too late anyway, I'm riveting HS parts now.

Jon, about the washing and acetone: my understanding is that for the primer to stick, two things must be achieved (1) a clean, oil-free surface, and (2) a rough surface. To get (1), I wash with dishwashing detergent and acetone, and to get (2) I lightly scuff with gray scotchbrite pads. It's not clear to me that I need to do (2) when using self-etching primer, but I do it just in case. I don't know if either is needed when doing the alumiprep/alodine routine.

By the way CJ, my Captain Morgan's Rum is brewed by Sam Adams.

John

[captain_john]

Well, as far as thickness goes... you should be able to see sharpie pen marks through a light coat of primer. That is how thin it should be.

Sam Adams is cool! A fine Boston Beer! My personal fave is the Oktoberfest!

CJ

[avaviat]

I thought dishwashing soap was a detergent. Maybe it is both....I know too little about such things.

Maybe someone who knows can evaluate this:
http://www.aerospace.henkel.com/Images/ ... rep_79.pdf

As I read it, they are saying the stuff cleans and acid etches in one go, prepping the surface for alodining or priming directly. I've got to say that my experiments seem to indicate that the metal surface was exactly as clean when I did #79 alone as when I did dish soap+MEK+#79. I hope it's true, because it was a LOT less work.

[captain_john]

It is a detergent. Detergent washing it fine. We are removing oils. Detergent DI-solves oils. Perfect!

CJ