Measure your damaged area. If it is larger than a quarter, you will use epoxy resin for this repair, otherwise, use polyester resin. Slower-curing composites will have a greater strength. Unless UV Cured coatings are used.
Note: Resin cures best above 18 °C (64.4 °F) (65 fahrenheit) and in moderate humidity.
Note: Polyester resin is porous and may not be suitable for use in areas which will be frequently underwater.
Select your glass reinforcement. If your damaged area is extensive or is in a structural area of the object you are repairing, you will need to use some form of glass in the repair itself. Smaller repairs that require strength reinforcement may use fiberglass filler, otherwise use fiberglass cloth.
Clear loose pieces from the damaged area and clean the area with acetone.
Tape off the area to be repaired with masking tape.
Mix resin and hardener, in the proportion recommended on the packaging, for a total volume twice that of the area to be repaired. Use the cup and stirring utensil.
Caution: avoid contact with skin, use eye and breathing protection.
If using fiberglass filler, mix the filler into the resin until the consistency is like peanut butter.
If using fiberglass cloth, cut a segment which will completely cover your damaged area and apply the resin to both sides of the material until it is just barely saturated with the resin.
If you are using resin without glass reinforcement or fiberglass filler, apply the resin until the entire damaged area is filled and just slightly overflowing with resin.
If you are using woven roving, apply the material until it is covering the interior of the damaged area entirely. If there are holes in the damaged area, these must later be filled with resin or resin mixed with filler as necessary (see steps above).
Note: If you have selected a hardening agent with a fast cure time, you may need to work quickly as you must properly apply the composite before it begins to harden.
Allow time for the repair to cure based on the recommendations from the resin manufacturer.
Caution: Curing composites are hot! DO NOT TOUCH curing composites.
Once cured, you will remove the tape and sand the damaged area. You may use course (40-60 grit) paper in order to obtain the general shape you desire. Once the general shape has been obtained, you will use a medium-grit (100-200 grit) paper to smooth the area followed by fine grit (300+) paper. You may use finer paper or polishing compounds until the desired finish is obtained.
Caution: Eye protection, skin protection, and a breathing apparatus are needed for sanding. Though cured epoxy may not smell noxious, particles created when sanding are still toxic.
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