Stevealex,
Don't be surprised when a supplier suggests that we use caution when coating prepainted aluminum sheets. Painting methods and paint technology for prepainted aluminum and steel sheet have changed considerably since I started in signs long ago.
In our shop we use acrylic polyurethane for most of what we paint, from foam to metal, and everything in between. It is durable and lasting, resistant to abrasion, moisture absorption, and fading. Polyurethane is the standard of the sign industry.
But the pre-painted sheet metal industry often uses paints that are even better than polyurethanes. Coatings made with
polyvinylidene fluoride resin,
PVDF are a significant step up from polyurethane. Kynar®, owned by Arkema, is a proprietary trade name for this resin. PVDF paints can do everything a polyurethane does—only better. In addition to resisting abrasion and water, these coatings offer high durability when being formed by bending, extruding, and die pressing equipment such as in the metal roofing industry, as well as in the sign industry. Unfortunately, PVDF paints can also resist top coating.
Many of us may be accustomed to simply hand rubbing the surface of pre-painted aluminum with a red Scotch-Brite pad before we paint. In my opinion, this is insufficient if the aluminum has been painted with a PVDF coating. I believe it is important to thoroughly flatten the gloss on this type of paint and then clean it well before top coating. My practice is to use 400 grit paper on a DA sander. 320 paper may work but you may get swirl marks in your finish. 220 paper is too coarse and 600 grit (or a red pad) will work but it takes longer. How do you know what type of paint your aluminum is coated with? You may not. Even your supplier may not know for sure. So, I simply assume that any prepainted aluminum that we buy is coated with a PVDF-based paint and I prep accordingly. Wrisco Industries, a long time supplier of aluminum blanks to the sign industry, sells aluminum coated with Fluropon®, which is a paint with 70% PVDF. Fluropon is owned by Valspar.
Aluminum Sheets - Aluminum Sheet Metal | Wrisco Industries Inc.
Paints with PVDF resins are great. They make for long-lasting aluminum signs and the prepainted sheets hold up well in breaking and bending operations typical to a sign shop. But they need good prep before top coating. I think it is wise to sand and clean even before breaking. You get a better sanding job that way and it's quicker. The old days, when you could just wipe down painted aluminum with lacquer thinner and spray it, are gone.
................................
How aluminum sheet is painted
High production painting of aluminum sheet is commonly done by a roller coating process called "coil coating." Aluminum sheet on a large roll is fed into a continuous coating line where the aluminum is etched, washed, dried, primed and top coated on both sides using large rollers, then baked in long ovens and quenched, all in a continuously moving process, to be coiled back up at the other end. Coil coating is fast, less wasteful and less costly per square foot than practically any other form of painting. Even anodizing of aluminum can be done this way. But the equipment is hugely expensive. And a coil coating line can fill a warehouse-size building. A large coil coating line can process several hundred linear feet of aluminum sheet per minute.
Brad in Kansas City
