[How-To] Priming Aluminum for paint?

randyjaco

randyjaco

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In most of my machine restoration I deal with steel and iron. My current project has several large aluminum castings. I tend to use Rustoleum products due to their easy availability. Rustoleum's aluminum primer is water based, it is difficult to spray and I hate dealing with it. The machine is a cold saw and is going to see a lot of coolant and abuse, so I am very concerned about the paint sticking.

The question is: Is there a specific paint and primer combination I should be using? My default is going with Rustoleum auto primer and standard Rustoleum paint. I would appreciate any advice in this area :)
 
You could see if this would fit the bill. It is my understanding that aluminum requires a different primer for adhesion, the original Zink phosphate was outlawed if I'm not mistaken.
www.ebay.com

Moeller Spray Can 025801 YELLOW Zinc Phosphate Primer Boat Aluminum Lower Units | eBay

Manufacturer Part Number: 025801. Boat Accessories: Deck-Cabin. Boat Inboard I/O Engine Parts. Boat Outboard Engine Parts. YELLOW color. Paint & Accessories & Tools. (below the price of the item). HAZARDOUS PRODUCTS (HAZMAT).
www.ebay.com
 
I have had good luck with the spray paints sold for painting aluminum gun parts. Direct to aluminum, no primer required. I used this to coat an aluminum floor vent I made, which gets walked on daily. No sign of paint wear in 2 years. It does require carefully following the cleaning and baking cycle instructions. The link is to a spray can, another common process is to use the cerakote brand products, which does require an airbrush.

Electro-anodizing aluminum would be the way to go if you had a lot of parts to do. You might check to see if there are any shops in your area that are set up to do it, probably not worth pursuing for occasional use.

Aluminum chromate coatings were the old standard for priming/protecting aluminum. ?May? still be used for painting aircraft. But it involved using hexavalent chromium products, nasty stuff, big cancer risk.
 
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Bonderite / Alodine are what the Army/Missile Defense Agency call for in their MIL-SPEC as both a preservative and adhesion primer for paint.

It’s a Chromate conversation coating and kind of converts the oxidized surface layer of aluminum so it doesn’t continue oxidation thus loosening the paint. (My understanding is all aluminum that is exposed to atmosphere oxidizes, so what you’re actually converting is that ‘shiny polished’ layer, which is actually oxidation, thus sealing in the rest of the aluminum?)

Fine sanding or blasting before Bonderite gives extra tooth for the paint to stick to. Bonderite has no real thickness.

It definitely has health risks so I’d prefer to brush it and wear protective gloves in a well ventilated area.

As @rabler said it is big in aircraft coatings. Apparently they *may* have switched from hexavalent to trivalent (or something like that) due to the super nastiness of hexavalent chromium???
I’m not knowledgeable about this aspect…

I am not an expert in this area but this is what I’ve picked up from asking questions of our paint division and engineers :p

There are a bunch of different flavors (think of the different formulas of Loctite) but this seems like a good option that I’m strongly considering exploring myself:

www.mrosupplyshop.com

BONDERITE M-CR 1001 AERO, 594693 ,$39.90 Conversion Coating,per Quart

BONDERITE M-CR 1001 AERO, Conversion coating per Mil-DTL-81706 Class 3
www.mrosupplyshop.com

Edit:

Just looked up the Henkel site…. Says Hexavalent right there!
So yeah… be safe if using it!

 
randyjaco said:
In most of my machine restoration I deal with steel and iron. My current project has several large aluminum castings. I tend to use Rustoleum products due to their easy availability. Rustoleum's aluminum primer is water based, it is difficult to spray and I hate dealing with it. The machine is a cold saw and is going to see a lot of coolant and abuse, so I am very concerned about the paint sticking.

The question is: Is there a specific paint and primer combination I should be using? My default is going with Rustoleum auto primer and standard Rustoleum paint. I would appreciate any advice in this area :)
Click to expand...
Are you talking about rustleum's etching primer?
I didn't think it was water based.

They used to use zinc chromate on airplanes as a primer on aluminum
 
Admittedly after dealing with two different forms of cancer, I'm paranoid. But the Alodine/Bonderite (chromate conversion coatings) just don't strike me as worth the risk. I actually have a bottle, still sealed of Alodine.

Looks like both of the rustoleum products are zinc phosphate based, the brush on in a water soluble solution, and the spray in an acetone thinner. I'd probably wait for suitable weather to spray outdoors ....
 
Thanks for all the replies.:applause 2: I am amazed at how little information there is on some of the suggested products. I never could find instructions for use on Bonderite, etc. They strike me as professional grade products that should require special procedures and applications.
So far, the Rustoleum etching primer looks like the safer choice; but I have no idea of how well the product works :chagrin:
 
Found a technical process bulletin:

Again, the health hazard is there. So it’s your decision.

But the shop floor knowledge (which isn’t always correct…) is just that you need clean aluminum. No corrosion or oil etc. You just brush it on (we used the pens for touch up mostly) and let it dry. Kind of like wiping something down with paint thinner.

The sanding/blasting only makes sure you have clean metal and a tooth for the paint.

Whatever you decide on, make sure you document it here for us! I know I want to see it :D

Edit:
Interesting, that document pretty clearly states that it should be water rinsed BEFORE drying to avoid salt powders affecting paint adhesion.
Hmmm….

That would definitely make things difficult. I wouldn’t want to wash that stuff off in the driveway and make your house a superfund site!
:eek:

As an entirely 3rd option… you could take it to a chem process shop that does it for you. It would be dunked in the vat and come out professionally coated. No idea if you have any ‘chrome shop’ type business around you or what the price would be. It’s another good option though!
 
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