This is a basic repair involving some glass repair and gelcoat work. The laminate, from simply looking at it, doesn't appear to be damaged. But after looking inside and seeing a fracture that you could hang your finger nail in, it became apparent that the glass itself was hurt. So after grinding away the gelcoat, we exposed the real damage. Lotsa seperation in the laminate. The grinding is best done with a good coarse disk. I like to use a 24 grit for this,the coarser the better. You can't do enough to insure a good bond. The area must be absolutely dry,and should be at a temperature of about 55 to 90 degrees fahrenheit with the humidity as low as possible. You can work hotter and you can work colder, but it can get tricky.
 

This a basic repair using fiberglass mat, polyester resin, MEKP catalyst, and gelcoat. It goes as follows, first the resin is catalized (catalyst stirred in) then it is applied with the brush to the part, liberally as you would in using paint remover,then a layer of glass, more resin, more glass, until you have the thickness you are seeking or just a very little more, then you use the roller to smooth out the laminate, compressing it slightly and removing the bubbles. Then it needs to cure. On repairs that require laminates thicker than one quarter inch it may be nessecary to do it in stages as the repair will generate some heat and could diminish the chance of a good bond.
 

This view is after the repair has been ground slightly with 24 grit disk and filed with 36 grit paper. Generally the higher the number, the finer the paper. At this point "fair" only means that the repair should conform to the part. You can develop a real feel for this with your hands. Just close your eyes and run your hand over the side of a car, or refridgerator for that matter, and if you use just a little to no pressure you will feel things you can't see. With practice you can feel every dimple, wave, ridge and dip in anything.
 
 
 

Once the repair is fair it's time to get to the gelcoat. the first application is done with a brush. The gelcoat has to be catalized also. Gelcoat is basically a resin with fillers and pigments to achieve a color wear surface. they can be matched as paints are, using their own type of pigments. This layer is applied as a filler for gouges, scratches and such. It is intended to be sanded off and should only remain in the low areas after sanding. We started with 36 grit and went to 80 grit and now are ready for the finish gelcoat, which is to be sprayed.
 
 

And finally a view of the finished repair. Between these last two pictures, the repair had been sprayed, using catalyst, styrene (thinner for gel and resins),sanded and buffed. Generally the gelcoat must be sanded with paper graduating from about 120 , 320 , 400 , 600 grit, and ultrafine, then buffed and glazed as was done in the olden days of 24 coats of laquer. There are many variations to the gelcoat procedures that we can discuss at length later.