Repairing Fiber Glass Hull Damage

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Many times when purchasing an old boat, there will be some fiberglass damage that you will need to repair. Many people do not like working with glass because they feel that it is either a hard detail oriented ability to master. However, glass work does not have to be hard, nor is it something that you need to spend a lot of time practicing to become a "professional" The only hard part to working with glass is matching the top layer to the boat’s hull or deck colors. The real nice thing about glasswork is that no mater how bad a person does it can always be fix by starting over.

The basic theory behind glasswork is to save as much of the area around the break or hole as possible. In the case of a sailboat, if the damage is where it can be reached from the inside, I try to save as much of the exterior finish as possible since that is a lot harder to make the hull look right then interior hull patches. If the hull has had a hole punched through it push the pieces back into position as much as possible, being careful to not break any more of the glass. Then start grinding along the cracks on the outside and feathering them back about an inch from the damage. Leave 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. Place the foam board 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.

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. The disks can be run in an electric drill, even a battery operated unit works well for small patches. Also a good tool for tackling bigger jobs is a small grinder like a Makita, but be careful these are powerful tools that can do a lot more damage in the wrong hands.

Once the grinding is done, cut small pieces of very fine glass cloth, 4 oz if you can find it. Start with a small piece in the center of the damage, and using polyester resin and a disposable brush start placing the cloth in position over the brake. Continue 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. Obviously if your whole is large you may need to do multiple layers between drying stages. Carefully trim off excess cloth with a single edged razor. If the resin was catalyzed correctly drying time usually takes about an hour to two hours. If there are small indentations or divots you can usually fill these with additional resin.

After the patch has fully hardened, I like to wait at least 6 hours sometimes overnight depending on the weather; you can then begin to sand. 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 are use 100 grit sand out about 1/2 inch around the patch on the good gel coat, you now are preparing for final coating, and color matching. Mask off around the sanded area and then using a small spray gun you can apply the topcoat. Gel coat can be thinned with MEK available at paint stores or Home Depot. 25 - 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 sandpaper. 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.
    4. Or finally the easy way, use clear cellophane wrapping like you can by at the grocery store.

The hardest part of doing the patch is to match the new gel coat color to the original. The technique I use is simply testing a match until I get the correct color that I want, then mixing the same proportions for the final cut. Of course you can figure that most coats will dry lighter or darker depending on the weather, this is where the experience comes in. However you should be able to do a reasonable job using tinting colors available from most vendors of gel coat, with a little patience. Remember that most gelcoat has faded on the hull and deck, so you can never trust the color match codes that the factory used.

Updated 07/04/2001 Version 1.5

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