I have yet to see a home painted pickguard that did not look like crap after a few weeks' use.
I think I can help prevent this. I've only done one pickguard, but I've painted a number of plastic automotive pieces. Musiclogic's advice is absolutely top-notch, but I'd stray from his method after the prep work - sanding, wet-sanding, and then drying thoroughly. From there, my recommendations are below. First, however, it's important to consider your environment. Fully enclosed, dry, warm, and dust-free are musts. Also, if you can hang the pickguard (coat hanger or other metal wire, routed through one of the mounting holes is perfect) vertically, you'll get much better results. Mix ratios with rattle cans are iffy at best, and trying to effectively spray with the can facing down is the first part of the recipe for bad results. Also, if you've got halogen lights, a space heater, or any other means of applying fairly direct heat, you can both speed up the process and shrink the curing time between coats.
Here are the materials I'd recommend:
- Painter's tape. 3M is great, but cheaper stuff will be fine too.
- Adhesion Promotor. Bulldog makes one that is both cheap and good.
- Automotive primer. Duplicolor is my favorite, but other brands are fine too.
- Automotive (poly) color. Try to match brands for the primer and color.
- Automotive clear. Ditto on the matching. Use this if you're after a very glossy look. It'll also protect the color coat a bit for more durability.
- Meguiar's Scratch-X or other mild abrasive polish.
- Meguiar's NXT or other automotive polymer sealant/wax.
- Microfiber towels.
And here's the process I'd use:
- Tape off the back of the guard completely. You don't want it to end up being any thicker than necessary, and taping off the mounting holes will make it very easy to remove any excess paint.
- Apply two light coats of the adhesion promoter. You can generally accomplish this in 10 minutes or so.
- Apply two-three light coats of the primer. Hit the guard with the space heater or halogens for 30-45 minutes.
- Apply three light coats of color. I'd wait 10-15 minutes between coats, and then hit the guard with the heat for 60 minutes.
At this point, let the guard rest overnight. You'll be anxious to proceed, but if you really want the guard to look good, wait.
If the color and gloss are as you'd like, you're done painting. If not...
- Apply two-three light coats of clear, using the same directions as above. Hit with heat for 60 minutes, and let it sit overnight.
- Using an edge of the MF towel, apply the Scratch-X to the guard. You'll want to work it in a bit so the abrasives break down completely, but do this more with speed than with pressure. It'll still take awhile...be patient and the results will be worth it. Once you're done, the finish should be very smooth and free of scratches.
- Apply a coat of NXT for protection.
- Pull the tape, clean the mounting holes, and toss it on the bass.
Good luck!