Regular mill finish aluminum has a oily feel to it
I want to add to Johnny Best's comment about the oily feel of mill finish aluminum.
CLEANING MILL FINISH ALUMINUM
Years ago I worked at a large electric sign company for a while and noticed the painter always washed the raw aluminum before sanding, and then washed it again after sanding and before priming. At the time I questioned his habit of washing twice. He explained that he believed that there were oily deposits on the aluminum when it came from the factory, and that if you sanded without washing first, the deposits could be ground into the aluminum by sanding, causing early paint failure. The second wash, of course, was to remove sanding debris.
Attending the Matthews Painting class since then has confirmed the truth of what this painter claimed. The Matthews instructor said that the manufacturing process does indeed deposit contaminants on the aluminum, and that sanding without an initial solvent wash can embed them into the surface and compromise the adhesion of the primer.
The instructor elaborated that the wiping method was important, too. He said that the wet aluminum should be wiped off before the solvent evaporates, otherwise the contaminants are not being fully removed. He said to look at the wiping cloth and repeat the wash & wipe till it no longer turns black. He also specified wiping in
one direction only, not back and forth or round and round. Wiping in one direction actually
removes the debris rather than just smearing it around. And he said change cloths frequently to avoid redepositing the contaminants.
To help keep the surface wet while you are wiping it off, the Matthews instructor warned against using a solvent that dried too quickly. He recommended a medium speed solvent that stayed wet till it was removed by the cloth. An example would be Dupont's Final Clean, or Matthews 45330 Speed Prep. Both of these mixes are heavy with VM&P naphtha, which has a medium fast evaporation rate.
I have made my own aluminum cleaner by using our gun cleaning solvent, which is Dupont 105 lacquer thinner, and added VM&P from the paint store to slow it down.
Another trick that is a time saver for quick aluminum cleaning is to apply the cleaning solvent with a three or four-inch brush instead of just misting it on with a spray bottle. I slosh the solvent on liberally with the brush and I'm able to quickly wipe an entire 4x8 sheet before any of it evaporates ahead of my rag. Applying cleaning solvent with a paint brush also helps when you are not cleaning a simple flat surface, such as cleaning brackets or screws, or cleaning a cabinet that has protrusions or reveals.
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PRIMING ANODIZED ALUMINUM
After I posted the information in my previous post above about the instructions in the Matthews Painting manual about completely renoving an anodized layer before applying paint, I looked at the section again on painting anodized aluminum. Matthews does
not recommend using an acid wash primer (like their PT Filler) after removing the anodized finish. They only recommend epoxy primer. No explanation is given, but this will be my next question for our Matthews rep.
Brad in Kansas City
