Repairing structural cracks involves several steps.
First, remove all wax and grease from the work area by wiping down the area with acetone and a clean rag.
Next, use a sander and coarse grit sandpaper to expose grind away the crack. Go no deeper than you must, and wipe down the area with acetone to be sure that the damage is gone. You may want to use some dye penetrant or paint to serve as an indicator.
Then, expand the sanded area to provide a minimum 12 to 1 slope down to the deepest point of your excavation. It is important that this provide a smooth transition, with no sharp edges.
Wipe the area down again with acetone to remove all dust.
Use low-tack masking tape and newspaper to protect the area of the hull surrounding the repair site. This will keep resin splatter and drips from becoming a problem for you later!
Cut several pieces of fiberglass cloth (biaxial weave) to the size of the repair site. You will start with a few small pieces (for the bottom of the excavation) and progress to larger pieces near the final surface of the repair. The top piece should be the largest, and some prefer to use chopmat instead of biaxial weave.
Next, add resin and catalyst into a mixing cup. Make sure to thoroughly mix the concoction. Use slow hardener here - we want the resin to have time to penetrate the existing fiberglass before it sets.
Brush a liberal coat of resin on the repair site and wait a minute.
Next, apply the smallest piece of cloth to the repair site and brush some more resin mixture on it. Use a fiberglass roller to roll out the air bubbles.
Wait a minute and repeat for the next layer of cloth.
Wait a minute and repeat for the third layer of cloth.
At this point, I wait until the resin is beginning to "gel", and then add 3 more layers of cloth using a fresh batch of resin mixture. These steps are repeated until the last layer of cloth is applied.
After the last cloth is applied, allow the resin to begin to gel, and then brush one more coat of resin over the entire repair. Let the work "rest" overnight.
Next, wipe down the repair area with acetone to remove the "blush" from the curing resin and lightly sand with 120 grit paper. Oh yeah - then wipe it down again with acetone!
Depending on your desired finish (paint or gel-coat) take the steps outlined in the
pages on finishing.
While they are very unsightly, structural cracks can usually be repaired to provide a hull that is as strong as new!
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