We paint trim cap once in a while. Since we use Matthews paints almost exclusively, we follow the protocols in the Matthews manual for painting trim cap:
1. Clean with 45330 Speed Prep Cleaner
2. Abrade with Scotch Brite pad (I usually spray glue a pad to the DA sander)
3. Clean again with Speed Prep
4. Apply one wet coat of any Matthews topcoat mixed properly with 47474 Flex Additive
5. 10-15 minutes to flash
6. Apply a second wet topcoat with Flex Additive.
The paint sticks very well. Matthews Tie Bond Adhesive (adhesion promoter for plastic) is not recommended in conjunction with Flex Additive.
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I always glue the trim cap onto the acrylic faces first, before painting it, but I usually scuff the trim with the DA before gluing.
It's very easy to mask the faces using Spraylat liquid maskant. In fact, I just brush it on, usually. After it's dry the maskant cuts easily with light pressure from a No. 11 blade. If the trim cap is already scuffed before you glue it, then you can paint as soon as you cut and strip the Spraylat off the cap. Any maskant that fills into small gaps where the trim cap ends come together imperfectly will pull right out cleanly. The maskant also helps to pull off any final debris or dust that was still on the trim cap after scuffing.
I could paint the cap first, before gluing—Flex Additive allows Mathews paint to bend without cracking—but I prefer not to simply because I'm afraid I'll get glue on the fresh paint when I'm capping. I tack the joint with Weld-on No.3 first, then follow it with a heavy bead of Weld-on No.16, and sometimes the Weld-on 3 dribbles onto the table.
You can paint screw heads at the same time you paint the trim.
Brad in Kansas City