Paint

[John Burk]

How well does 2 part primer stick to a bare aluminum body .

[Rick Byrnes]

Great, if you prep the bare alum right.
DA 180
Wash with aluma-prep and steel wool.  rinse thoroughly.  Lightly
spray with zinc-chromate primer.
scuff and prime with modern 2 part primer.
I used a 2 part top coat as well.  Only one year on the salt, but lots of time getting garage rash. (not)

I'm sure there are variations, but this was taught to me by my dad, who back in the day, was a very well respected local painter.
It works for me. 
No delamination at the alum interface.

[manta22]

I'll agree with Rick.

Preparation is the key, the aluminum surface needs to be scuffed lightly- I use an orbital sander- then cleaned with Dawn dishwashing detergent & water, rinsed well and then treated with a phosphoric acid etch. A chromate conversion treatment such as Alodine is nice but not necessary if it will be primed.

I wipe the aluminum surface with denatured alcohol to do a final clean and then spray on a two-part zinc chromate epoxy primer. It cures overnight into a smooth hard finish. Top coating is optional. Check the Aircraft Spruce website for materials.

Regards, Neil  Tucson, AZ

[John Burk]

What results good or bad has anybody had with less thorough prep than Rick and Neil described .

[wobblywalrus]

Experience has taught me to scuff off the oxide just before before priming and to wipe the surface clean, just like Rick and Neal.  Bronze wool with acetone or alcohol or lacquer thinner is what I use the most often for the scuffing.

Acids work OK for removing the oxide, too.  They can be more trouble than they are worth if they get onto surfaces where they should not be.  This is a problem when you are doing a spot repair and the nearby surfaces are painted.       

[kiwi belly tank]

About the toughest conditions for painted aluminum to survive in is on a boat. A bunch of years ago I built an aluminum jet boat & it was going to cost me more than my cheap-arse budget could afford for the correct paint. So I scuffed it up with a kitchen pad, washed it with water, mixed a pint of water based primer with a gallon of vinega & squirted the bitch then painted over that. Basically just a poor mans etch primer. I had to grind through it to be able to repair hull damage after that, it stuck like $hit to a blanket.
  Sid.

[manta22]

John;

My worst paint job ever was the time I painted an aluminum rack panel when I worked at Green Bank--  no surface prep at all. It looked clean so I didn't bother. I painted it with a spray can of GSA gray paint (el cheapo government issue stuff) and the next day I used an air hose to blow off some dust-- it also blew the paint off in small sheets. Lesson learned.

Regards, Neil  Tucson, AZ

[gas pumper]

I've done lot's of aluminum over the years too and did all the above procedures but never did the phosphate wash. But always used two part zinc chromate primer. And never had paint peeling off the bare aluminum.  But Now I would take the extra step and do the metal prep wash. (I have been doing the metal prep wash on iron, steel, sheet metal and like the results better) 

Frank

[Rex Schimmer]

John,
I assume that all this "paint talk" means that we are going to see you liner at the salt this year??

Rex

[John Burk]

Thanks for asking Rex , yep looks like I may make it this year . Incidentally no body has answered my original question about how well 2 part primer sticks to well cleaned aluminum .

[tallguy]

John, you've apparently changed your question.  Regardless, though, if
you're not getting the "easy" answer(s) here, you may have to (gasp!)
research it yourself.  I suggest you ask professionals in this field, or
go to websites (i.e., forums) of people who specialize in this.

By the way, my personal experience with painting on aluminum is very
limited, but here it is (for what it's worth).  In 1979 I wanted a sign for
the back of my pickup truck.  I bought a rectangular sheet of aluminum
about 1/8" thick.  I don't remember the alloy.  I just degreased it a few
times with 1,1,1 trichloroethane (it was easy to buy back then, but is
apparently carcinogenic) on paper towels.  I didn't do any sanding or
other abrasive prep.  I took it to a paint shop and told the guy that I
had just used the 1,1,1.  He tested the aluminum for cleanliness by using
a piece of masking tape which stuck pretty well, and said "that's good
enough."  I don't know what further steps he took to prepare the metal,
but the white paint (background) and black lettering lasted for at least
ten years of driving around and being parked outside (NEVER inside!).

Good luck.  I would hire a pro for this if I had the money.

[thundersalt]

Thanks for asking Rex , yep looks like I may make it this year . Incidentally no body has answered my original question about how well 2 part primer sticks to well cleaned aluminum .

It won't....You HAVE to use zinc chromate. Wipe your hand on bare alum (even after cleaning), look at your hand, it is black. That is oxidation. Like any surfactant, that keeps topcoats from binding to the surface. Zinc primers are formulated to bind or even coalesce with the oxidation to bind to the surface........I am not a chemist but you get my point

[John Burk]

In the 60 when I had my body shop friends paint my aluminum dragster bodies they probably were the first aluminum they had painted . They just wiped them down with Prepsol and primed and painted . One of them I had the body anodized and painted sections . Never had any sign of poor adhesion , even with the anodize which is a heavy oxide layer . Logic would say that modern 2 part paint would stick better than 60s lacquer . Being a naturally thrifty person I'd rather not use heat treated nitrided 4340 where mild steel is sufficient . Would still like to hear from anybody who has just degreased and 2 part painted aluminum .

[thundersalt]

Well, I have (being a RV collision repair shop we paint a lot of aluminum). A few times when something has to be done and we're out of zinc we've used a catalyzed primer only and you're right. As you said it sticks a lot better than lacquer primer. But after time when chipped or scratched it starts coming off. In any form of painting there is the right way and the wrong way. You may get away with cutting corners sometimes but with the cost of primers and topcoats, not to mention all the sanding and prepping which no one enjoys, why not zinc primer first? It only takes a piss coat, maybe a little scuff pad before you continue with other products usually within 20 minutes.

[USFRAMONTE]

I'll throw my experience and opinion in here to see if I can muddy the water a bit.  Up until the mid eighties, most all paint had lead in the formulation.  The lead
helped in both adhesion and hiding (how opaque the color is).  The lead is now gone and with it a great deal of the adhesive properties of the "old finishes".  I agree
with Thundersalt, with the cost of the labor and materials for a paint job nowdays, it is best not to short cut or skip any of the recommended steps, and especially the
recommended mixing ratios.  Also, it is best to stick with one manufacturers products from start to finish.  That way you are using chemicals that are engineered to
work together in the best way possible.