Some shops may struggle with fiberglass hoods/bodies shrinking back to the point you can actually see the fiberglass weave pattern in the paint a few months/year later, waves appearing that were never there before, or even blisters. The question is WHY?
Step 1 in Fiberglass parts Preparation is a good long bake. Most fiberglass hood and body manufactures recommend baking the parts in the booth in a bake cycle for at least 1 hour. we have found that the Infared Light works much more effectively for this process. As we talk about quite often, the Infared light will actually heat the panel from the bottom-up vs whereas a booth bake will be from the outside in. most often you will bake the panel at a surface temp of 150-160 degrees F for at least one hour. its always a good idea to go the extra mile. bake it for an hour, let it cool naturally, bake it another hour, repeat as many times as you like. the more you bake the less likely you are to have shrinkage later.
When fiberglass parts are made they use a liquid resin, activated by a liquid hardener (peroxide mainly). this liquid product like many liquid products contains high amounts of solvent. when these parts are made there are solvents in the fiberlgass, even after its " cured". These solvents are the reason that some fiberglass parts shrink so much.
When fiberglass parts shrink, they often form new waves in the panel or you may even see the strands or weave of the fiberglass showing back through in the paint. this happens because as the fiberglass (specifically the resin) dries it loses mass like any other product. Shrinking is natural in our industry, every liquid product WILL shrink, the question is to what extent. When the resin loses solvent and shrinks it will pull the topcoats (gel coat, primers, paint) down with it, magnifying the effect. some people will try to combat this with a high build primer or spray poly, which is NOT the answer to shrinkage.
Once your panel has been properly baked you can then begin body work. You may even notice the panel forming a wave or blister that was not there before, this is due to the shrinkage that we caused and combated before body work with the infared light bake.
Why do Blisters form in Fiberglass parts? In my experience when a blister forms on a fiberglass panel it was because there was a large pinhole in the fiberglass close to the surface. Typically gel coat would combat this but sometimes you may be blocking dangerously close to burning through the gel coat, and of course that is the area where a pinhole will be lurking beneath the surface.
So why does it not blister right away? You may paint the part, it looks great, it leaves. Months later it comes back with blisters. This is because that large pinhole is a small compression chamber at this point. when the air in the pinhole is heated (usually by the sun) it will create pressure as the trapped air expands. if there is minimal strength above that area, POP! the pressure will literally break the substrate surface forming a blister. This is another advantage to baking the parts before blocking/filler work, after body work before priming, and after priming. If this problem will arise, it should happen before paint during one of these bake cycles