Fibreglass is a “fluid” material. It will form or move under tension. For example, I riveted in the anchors for my rear bonnet supports. Over the years since that area has reformed due to the stresses in play. FG also expands and contracts due to heat/cold more than metal. If you have irregularities in your fibreglass, they will eventually show through. The only repair method I have seen that helps reduce show-through is the use of “veil” over repaired sections and imperfections. This gives a consistent surface to refinish on. It will move around and expand/contract consistently and much less likely to affect the finishing coats. Without the veil coat, imperfections in the original fibreglass (stress cracking) will have it expanding/contracting in different directions. This causes the stress lines in the fibreglass to “appear” in the finish layers.
My car was done by a very good shop, well versed in working with the heavy, thick fibreglass used in Corvettes, etc. They had no experience with the crappy, irregular fibreglass created by Lotus. The car came back looking absolutely, unbelievably stunning. As I put it together, it went from outside from time to time. Body heat cool cycles gradually had any stress lines appear in the paint underneath the clear coat. It’s only visible in certain light conditions, most people still rave over the finish — at least until you point it out.
I was quite disappointed and the shop was shattered. We came up with a happy compromise — they did a bit of free work for me and we shook hands and called it even. My car was always meant to be a driver and drive it does. 5K trips happen once or twice a year and in all weathers. I still wince a bit when I see the “cracking” but otherwise give little thought to doing anything but enjoying the car.
However, NEXT time, the whole body gets veiled, no ifs and/or buts about it!