Bringing in a post from my build thread:
I also recommend checking out the youtube channel "B is for Build" and watch the series on rebuilding a Lotus Evora. He shows a pretty good example of his learning curve with fiberglass repair and some of the mistakes and adjustments he made in the process.
Posting videos for reference:
Just watched it, and I would suggest some caution when using that as an example. He made some pretty big concessions, I'm guessing because he was in a rush
There were a few major things I noticed and he mentioned some of them.
1. Using rattle can primer. This is the foundation of your paint job literally everything you do rests on the adhesion of that primer and the surface you create with it. I would rather apply good primer with a foam roller, or paint brush than cheap primer from a spray can. A 2k primer is all you'd ever want to use outside of a race car paint job, or some cheap job on a beat up honda. Google your local automotive paint supply shop. They accept walk ins, so you don't need to be a professional to go there, and ask the guys behind the counter what the good stuff is. What are the professionals buying and using. They'll tell you, most of em have worked in the industry a long time, and will basically give you free advice on how to do the bodywork.
2. He didn't mention the stripper he used but I'm assuming he would have mentioned it, if it was specifically designed for fiberglass. Certain strippers, as he alluded to will literally eat away at your fiberglass. This is a massive potential problem, and you should always be aware of the solvents and strippers you're using on fiberglass. It can cost you your car.
3. Adressing the cracks up front. He handled the primary crack just fine, same as I mentioned above, and he tried to dig out the spider cracks, which is smart, because trying to grind them all out and create massive craters everywhere can get REALLY ugly, REALLY quick. But he just adressed the major spider cracks. There's no way he got all of them. When he zoomed in they were everywhere on that bumper. Basically the whole surface 6" to a foot from either side of that big crack was all spidered paint. He should have at least taken some 80 grit across that whole area, sanded it all the way down to bare fiberglass done the same on the back, and then shined a flashlight through the panel to illuminate the cracks. You could also probably use guide coat to illustrate the cracks too, but just the act of stripping the panel probably would have taken care of most of them, and there's probably dozens more small ones he never touched. Those I would either continue to sand the fiberglass with 80 grit until I sand through them, or if they're showing up all the way through I would continue to dig them out / cut them out. Then if you've taken a lot of the thickness off the panel grinding them out, you need to lay new fiberglass over it in as smooth of sheets as you can manage. And this would be a lot of work, because now you're rebuilding the surface of the panel, and you've got to re-contour it to make it smooth and straight again (read block sanding which is a pain in the ass, cant do it with your powertools). Also right before he was about to lay paint he pointed out a section that might have been painted with the original white, or it was bare fiberglass. Thats kinda a big difference, you should have primer everywhere. Bare fiberglass can suck in the paint and that section will look odd down the line when it dries, plus potential adhesion issues. Same with the rear wing, you can't just start spraying primer until you're surface is uniform, or else "wierd **** happens" as he said.
4. Which brings me to 4, where;s your guide coat. If you're not using guide coat and you're trying to spot highs and lows (as a noob anyway, I'm sure experts can get away with lots of stuff) you're completely flying blind. So for every high or low spot he mentions there's probably scratches and other spots that you'd never see.
It seems like overall he was mostly in a rush, so that probably explains it, but this process I think would get you equivelent to a cheap paint job, maybe a bit better than a Maaco style. For cars this nice, I wouldn't do that. It takes a lot more time to do it right, and I mean A LOT more, and its nearly impossible to do without a DA sander, and a Mini DA sander. I picked up the mini 2-3" DA sander from amazon for like $60 and it's a beast. Thats what you'd use to attack those areas he said were too tough to reach.
Also despite us living in the 21st century, there still happens to be a MASSIVE difference between cheap and good sandpaper. Like 5-10X difference. When my painter told me to buy 3M Cubitron II sanding discs and I spent nearly $300 on sanding paper alone I cursed him out. When I tried sanding the panels later with cheap paper from amazon and my paper was getting totally loaded up every 20 seconds I understood where he was coming from.
BTW loading up the paper is what happens when you try to sand surfaces and rather than falling off as sanding dust, that residue starts to collect on the sanding paper and fill in the abrasiveness of the paper so it stops working. Some paper sheds dust shockingly better than other paper instead of collecting it. Also has to do with what you're sanding, if it's soft and gummy it won't turn into dust that falls out of the way, it turns into gummy black garbage that coats the surface of your paper.
Sanding is the #1 time suck in prepping for a paint job so spending the money is worth it always.