There's basically two types of GelCoat... see below...
Gelcoat is applied to a thickness of 20 mils.
If a laminating gel coat is used (no wax), no sanding is required between the applied layers, as the gel coat cures to a tacky surface. However, for the final coat, a Mold Release (typically PVA) must be sprayed onto the surface as the gel coat begins to gel. Mold Release agents create a thin film on the surface of gel coat. This film seals off the air and allows the gel coat to cure to a hard finish. The Mold Release agent can be washed off with soap and water after 24 hours.
If a finish gel coat (contains Wax) (One Step Finish Gel Coat) or polyester gel paste is used, sanding is required between the layers. This finish gel coat cures to a hard surface, which needs to be roughed up to insure proper adhesion. Use 200-grit sandpaper to achieve a proper surface. The One Step Finish Gel Coat and / or Polyester Gel Paste do not require the use of a Mold Release agent to achieve a hard finish.
Once the gel coat is cured, sand with 400-grit sandpaper, then 600-grit or finer to achieve the final desired finish.
The surface must be dry, and as smooth as possible. Sand the area with 80 grit. This gives the Gelcoat something to "Bite" into. The final gel coat layer will look only as smooth as the original surface was. Remove all paint, grease, oil, dirt or wax. Wipe it down with acetone and allow to dry prior to applying the first layer of Gelcoat.
