I have sanded off all of the paint, primer and gelcoat from my 74 using a DA sander with 80 grit. I had a lot of spider cracks in my finish before I started. I found that the spider cracks were not only in the finish color coat, but in the primer and gelcoat. Not all of the body had gelcoat, just the engine cover and doors - this gelcoat was powder blue in color. Other parts may have had a clear gelcoat, but I never noticed any. I think spider cracks can be eliminated by sanding back to bare fiberglass and I think the spider cracks are caused by surface stress - like around door handles and locks or from impacts like some inconsiderate soul banging the body with their car door in a parking lot. For the paint finish, I start with (2) coats of 2-part black epoxy to seal the fiberglass surface, then sanded that lightly with 180 grit to get rid of any surface flaws, dust particles, etc. followed by (4) coats of high build polyester primer/surface. I then sanded this with a long board and 180 grit using a black powder guide coat to reveal any high or low spots. If any low or high spots appeared, I applied another (4) coats of the high build polyester primer/surface, 180 grit and guide coats and repeated this process until I achieved a perfectly flat surface. I'm still working with this process in preparation for applying a single stage urethane color coat. In what appeared to be high stress areas, I applied (3) layers of 3/4 oz. matt fiberglass to the back side of the body to strengthen these areas - like around door locks or areas that were really thin. Some of the fiberglass is so thin that you can see a shop light when it is held behind the body. Needless to say, this process is really time consuming, but I find it works.
Tom
74 TCS - 4605R