Painting over a powder coated metal surface can be frustrating because it is difficult for new paint to stick to the old coating. Most paint will not stick to slick surfaces. Special preparation and compatible primers are required for success.
I recommend cleaning powder coated metal in the same manner that you would clean a vehicle before applying vinyl graphics.
First, wash the surface with detergent cleaner to remove water based contaminants. Then clean the substrate with a wax and grease remover. The third step is to wipe the surface down with isopropyl alcohol (IPA).
Powder coated surfaces are very slick so paint does not have much to grab onto. To give the surface a little tooth, lightly sand it with 180 grit sandpaper or a red Scotchbrite pad. After sanding, wipe the surface down again with IPA.
If the powder coating has chipped away from a steel surface and it has started to rust, simply sanding is not enough to ensure good adhesion. You need to sandblast the substrate to remove any rust prior to priming.
Test, Don’t Guess
Powder coatings are high molecular weight polymers. There are many different formulations of powder coatings. Because of the different chemistries, what sticks to one coated surface may not stick to another. This is why I suggest that you “Test, Don’t Guess”.
Before proceeding with priming and painting, first make sure that the primer sticks. After cleaning and sanding the surface, apply a small spot of primer. Wait for the primer to dry. An hour of drying is usually sufficient. Then try to wipe off the primer with a rag. If you can remove the primer or it smears, it’s back to the drawing board.
Prime the surface with XIM UMA® Primer Sealer. The old time sign painters would use this product when they were repainting a porcelain sign or painting on glass or ceramic, all of which are slick surfaces.
After priming, paint with sign enamel, such as 1 Shot lettering enamels.