As you pointed out this got me stuck in this cycle of constant sanding and adding and sanding etc.
Apparently you're still stuck:
I just wasn't happy so used some 80 to remove the skim type coat and then stepped up to 150 and then 320. Shot with some high build primer in some areas to see what I could smooth out and then this morning used 80 to clean it all up... then stepped up again and shot with a few coats of normal primer. Tomorrow or I guess later today I'll look it over again, fill or sand any little areas that need to be fixed and reprime though areas.
There was no need for 320 grit until much later in the game. All contouring (working with filler) can be done with 80 and 120.
High build comes much later (see below). The only thing that goes on over high build is finish paint; the color and (if desired) clear coats. What you're doing above is the "layers and layers of mixed stuff" previously described, which is bad procedure on several levels.
There is no reason to spray anything until the part is entirely filled, contoured, and sealed....a done part. Up to that point, use nothing but a quality filler. Opinions vary, but I'd stick with epoxy and glass micro over an epoxy matrix composite. Polyester/micro/talc/styrene premix auto body filler is fine over polyester matrix composites or gel coat, or just use epoxy/micro there too.
The internet is full of advice about spraying guide coats so you can see the low spots. Maybe it's necessary if you're working by firelight in a cave. For the rest of us, a few pencil scribbles does the same thing, and doesn't contaminate the surface with mixed chemistry, or clog the sandpaper. You don't even need the pencil if the part can be viewed with flat lighting.
See the shiny spot in the middle of this panel? It's a low spot where the sanding block has not cut. It needs filled, or the surrounding material needs cut down to the same level.
After the part is entirely on contour, you'll need to kill the pinholes. I like an epoxy skim coat, sanded slick without breaking through. Some guys are using drywall mud, squeegeed into the surface and sanded until no trace of it remains except down in the pinholes. Azko sells a special pinhole fill product, as does Lohle. The drywall mud, Azko, and Lohle methods are all pretty much the same concept; after application, the product remaining down in the pinholes is locked into place by a subsequent application of primer. However you do it, fill pinholes before spraying any kind of primer.
High build primers are used as a final step, just before color. Their purpose is very fine adjustment of surface contour so it is optically flat, i.e. no waviness is apparent in the gloss-painted surface when viewed at a low angle. High build is not applied until after the part is entirely contoured and sealed. If you measured the surface contour of the bare part, the highs and lows would be perhaps 0.005" difference, maximum. That's the build thickness in "high build"...a few thousandths. For example, PPG K36 is sprayed in 4 mil coats (0.004"). After cure and blocking, the desired dry film coat is 1.5 to 2 mils (0.0015 - 0.002"). If you have low spots measuring 0.003 deep, it will take two or three rounds of high build and block sanding to get it perfect.
How to use it? With the surface entirely contoured and sealed, the best approach is spraying a dark color epoxy primer followed immediately by a lighter color urethane high-build. The wet-on-wet approach ensures that the urethane high build is "glued" to the sanded epoxy surface, and the color contrast is a sanding guide.
The cured high build is block sanded with hard blocks, using 400 grit wet-or-dry paper. The idea is to skim off the high places and leave the lows untouched, thus the hard block. Sand evenly across the entire surface until the dark epoxy primer just starts to peek through at the high places. Stop...do not cut into the dark primer! Spray another 4 mil cross coat, cure, and block again. The process repeats until blocking the entire surface does not result in dark epoxy primer showing through anywhere, while the former lows are now built up enough to be cut by the sanding block.
Glass canopy frame, at the high build stage. See the dark primer peeking through at the highs? Time to spray another cross coat of high build.
I should work on molds, not sure what I would use for a reverse type mold.. maybe cut foam that I can then push down onto the top.... I tried using peel ply a few times to push filler into a shape, worked somewhat descent, still had a ton of sanding to do.
Female molds and male forms have nothing to do with finishing factory supplied parts.