Repairing Fiber Glass Hull Damage
Contrary to what many believe the repair of fiberglass is not difficult The real nice thing about glass work is that no mater how bad a person does it can always be fix by starting over.
On of my primary philosophies is to save as much original hull as possible. In the case of a thistle if the damage is where can be reached from the inside I try to save as much of the interior finish as possible since that is a lot harder to make look right then exterior hull patches.
If the hull has had a hole punched through it push the pieces back into position as much as possible and then start grinding along the cracks on the outside and feathering them back about an inch from the damage and leaving the edge next to the crack or tear as thin as possible while leaving the inside intact as much as possible. As the edges along the tear or crack become thin the glass will tend to return to its original shape. If the pieces won't stay in place use a piece of urethane foam insulation board and place on the inside to push the hull edges out to the correct position. Sanding the insulation board to match the shape of the inside of the hull will allow pushing the hull back into the proper position. More on the use of insulation board later.
For small repairs the best tool for grinding is a small 2inch diameter 3m disk on a mandrel. These disks have a plastic screw on the back that snap into the mandrel. They are available from automotive paint stores and in some marine catalogs. Also Mac tool dealers and Snap-on Tool dealers can usually get them for you but the cost is a little high. These can be run in an electric drill, even a battery operated unit works well for small patches. 36 grit works well to do most of my grinding.
Once the grinding is done cut small pieces of very fine glass cloth, 4 oz if you can find it and start with a small piece in the center of the damage, and using polyester resin and a small disposable brush start putting the cloth in position over the brake. and keep adding layers that are a little larger then the previous until the thickness is just above the original hull surface. Do this all in one operation and then let it harden until it can be cut with a razor blade without puling fibers free. then carefully trim off excess with a single edged razor. If the resin was catalyzed right for the temperature this should take about 45 to 120 minutes. Then using a small sanding block finish leveling the patch to the surface. I usually start with 80 grit and then move to 100. If there are any hollows or divots left after this use a fine body filler to bring level and sand again until surface is level and contours match original.
When the repair area is sanded level and matches the contour of the original hull us 100 grit sand out about 1/2 inch around the patch on the good gel coat. Mask off around the sanded area and then using a small spray gun or a small disposable sprayers available in most paint stores the area. Gel coat can be thinned with MEK available at paint stores. 30% MEK usually makes the gel coat very sprayable. Polyester resins such as gel coat do not totally cure in the presence of air and the surface of the sprayed area will remain soft and quickly plug up sand paper. There are several solutions: 1) shortly after spraying the gel coat spray over the patch with Poly vinyl acetate (PVA) which creates a water soluble film that seals off the air and allows a full cure. 2) Add a surface curing agent to the gel coat, I have found this changes the color and texture of the material and patches do not disappear as well. 3) spray the gel coat extra heavy and after it has cured wipe the surface with acetone to remove the uncured surface.
The hardest part of doing the patch is to match the new gel coat color to the original. My technique won't work for you since I delegate this to my artist wife who does and outstanding job of making colors to match. However you should be able to do a reasonable job using tinting colors available from most vendors of gel coat.
There are also companies that you can send a sample of the original gel coat to and they will do the matching This requires about a square inch of the original gel coat.