I have recently been prepping a fiberglass body for paint. There are a few things to consider when choosing a body to buy. First you want to find a reputable company making the bodies. You can find a cheap body but you will get a cheap body, where nothing fits. Trust me i had a guy bringing me a cheap body and it took about 500 hours to fit the doors, hood and deck lid. You must find a good body to start with even if it costs more, this will save you money in the long run.
This first thing I did bolt up all the body panels. They fit pretty close but they needed gaped. This will take a lot of time but it has to be done. I had to sand on edges and build up some edges. I used fiberglass matting and resin. This ensures the repair will last a long time. There are a lot of fly by night shops will just use body filler but that will crack and break in no time. I used a paint stick wrapped in 80 grit to sand all the gaps even. Now that the gaps are even you have to make the exterior panels flush. This will take a lot of sanding and building with fiberglass. I block sanded across all gaps until every panel was flush.
Once all the gaps are set you can straighten the main body. This will take a skim coat of a quality body filler, (I use rage gold). Skim the entire car and block sand with the longest block you can. I use a Dura Block that bends to the contours of the car. This method works great. A lot of people say "Well I do not want filler in my car" so I ask them "so you do not want a straight car?" The car must be skim covered and block sanded, the only way to get a laser straight car.
After the rough block sanding it is time for primer. I always use Dupont Chroma Premier primer. This is a high quality primer that hold out very nice. (for a high quality paint job do not skimp on products, it's not worth it) Put about 4 wet coats of primer on the car and let it cure for a day or two preferably in the sun. Now its time for more block sanding. Always use some type of guide coat. Rattle can paint works fine here just spray it on lightly and that shows you your high a low spots. Pin holes are always a concern on fiberglass bodies. Look very close and fill any pin holes with filler making sure to push it in the pin hole with a razor blade. We always do this process at least 2 times with the body all bolted together blocking across all the gaps. Once it is straight I break the body down and prime the body one last time hitting the jambs as well. Now you are ready for final sanding.
Final sanding is a very tedious part of the paint job. This is where you can sand a line in your paint job. Very carefully sand with a block about 12 inches long with 400 grit paper wet. Always sand at 30 degree angles holding your block perpendicular to the body panel. Once it is all blocked out wet the car down with prep-sol and sight down the panels looking for waves. If there is a wave you have to go back to the body work phase. Finally guide coat the car again and final sand with 600 grit paper wet. Now you are ready for the paint of your choice. We use Dupont.
This is just a tip of the work that goes into painting a fiberglass body but it gives you an idea of ??what goes on. This is why show quality paint jobs can cost anywhere from 10-20k. The amount of work that goes into them is unbelievable. The last one we did took about 750 man hours.