If i sand off the gel coat, do i need to apply epoxy and 410 over it or can i just primer/filler, sand, prime and paint?
Depends on how good you want it to look when done.
In general, working from bare epoxy/glass outward...
1. Contour - Goal is to set the
shape of the part. This is where we use a bulk filler like dry micro, i.e. epoxy and microballoons mixed to something like firm peanut butter. As a rule, apply what seems like too much micro, rasp off the excess when it cures to the cheese state, then sand to contour after cure. The idea here is to apply micro
one time. It's a common rookie mistake to apply, sand, find a low spot, apply more, sand, repeat. It doesn't work well because the multiple applications rarely contain the same ratio of epoxy to balloons. The sanding density is different, so the surface won't sand out level. One fat application guarantees consistent density, so it block sands level.
2. Seal - Both the epoxy/fabric and the cured/sanded filler will evidence pinholes and porosity, as well as 60 and 80 grit scratch marks. I prefer to seal with straight epoxy, wiped on with a squeegee, then rolled to eliminate drag marks. After cure, block sand with 120 or 180 grit. Which to use depends on the next step, 120 if subsequently spraying a primer/surfacer (see below), 180 if going directly to epoxy primer and topcoat. The later is mostly for small parts, like an intake snorkel.
3. Optically level - In the end, we want to be able to sight along large flat panels and not see any waves in the surface. Given low angle lighting, a wavy surface is ugly. To ensure a level surface, we spray a primer/surfacer, in essence a urethane primer containing a fillers like talc and styrene. It builds surface thickness, which is then
block sanded for flatness. Block sanding is just what it sounds like, sandpaper on a hard (or quite firm) block...no sandpaper held in the bare hand. The goal is to cut the high spots while bridging across the lows. We're talking a few thousandths here.
Contouring was back in step one.
Product lines vary in application details. For PPG, spray DPLF epoxy primer, the follow in 30 minutes with two cross coats of K36. Use a dark primer under the light primer surfacer, for contrast. When cured, block sand wet with 320 or 400. Work evenly with large strokes and the largest block suitable for the panel shape. When you see dark primer showing through the high spots, stop sanding, dry, wipe clean, and spray two more cross coats. Repeat the spray and blocking process as needed until sighting along the surface with low angle light shows no waves at all.
Blocking is an art form at the best shops. Here's a great example, Monty Barrett's 40 Ford. No waves in those body panels. Check the reflection in the left rear quarter.
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