A bunch of us shade tree body guys chiming in but at least we are consistent.I’m by no means a body guy but the half dozen or so vehicles I’ve needed filler on (and another dozen motorcycles) I’ve always used rage gold.
I use thin, smooth cutting boards from the dollar store to mix it on and spread it from. Don’t mix too much because when it starts to kick, it’s done and just toss it. Better to mix a few baseball size batches than one big batch that hardens while you’re trying to work it.
With the thin cutting boards (like 1/8”) you can feel the heat from the chemical process. Pretty crazy.
I typically went with the Evercoat products like Rage because that's what the shop bought. I also used my fair share of 3M on personal stuff. But yeah, it really is about getting the mix right, if it's a new product, playing with it, and taking your time.
Beat the metal back into shape, a coat of bondo, sanded and then a top coat of a glazing putty to fill any pin holes.
I should also note since this sort of aligns with wheeling - I used Evercoat Everglass on all my lowriders and mini trucks. Yes, I'm telling on myself here. They typically had air ride or hydraulic suspension and would be put to some serious flex stressing. Everglass has small fiberglass strands. Standard stuff would sometimes pop if you were lazy on you metal work.