I am in the middle of body work on my 74 TCS. I have removed (3) coats of primer and (3) coats of color with a DA sander - did not find any evidence of gelcoat - if it was there, it was clear. Like many, I had a lot of stress cracks in the old paint finish. After removing all of the primer and color down to bare fiberglass, all of the stress cracks disappeared. I have been told that wiping over the stress crack area on bare fiberglass would show up the stress cracks - I tried this and still no stress cracks. My car had been repainted (same color) twice before I bought it in 1978 and I think the paint prep work was probably not the greatest and the paint was applied too thick and not enough time was given for the primer to thoroughly dry (like a month) before the color was applied. Thus far, I have applied black epoxy primer and then high build polyester primer/filler. I've also done panel fitting to get the panel gaps between panels even (4.5mm gap) and the adjacent panels meeting in the same plane. I'm working with PPG paints and primers. I'm doing block sanding now. I plan on using PPG single stage urethane color - no clear after applying the color. My belief is that most, if not all, of the stress cracks are in the paint itself and not the underlying fiberglass body structure.
Tom
74 TCS - 4605R