My hat's off to you for tackling the bodywork on your own. I'm brave enough to try some things, but not that. Looks like you're making good progress. I believe 3/8-3/4" thick Rage Gold is maybe too much. But I'm not an expert on the subject at all. Maybe one of the Jeff's will weigh in. At the shop that did my Coupe, the guys did mention the area behind the side quarter windows where there's a pretty poor mold match they used some type of resin material for the fill. Said it would have been too thick and not strong enough with regular body filler, e.g. Rage (I don't know what brand they used). The material you're talking about is affectionately called kitty hair. Resin with strands of chopped fiberglass. I don't know if that's what my guys used. I do know there is some concern with using materials that are compatible with the vinylester resin FF makes their bodies from. Good luck and happy sanding.