How to Repair Fiberglass Finishings on Boats, Cars and other Objects

How to Repair Fiberglass Finishings on Boats, Cars and other Objects
Author Info | Reader-Approved
Updated: August 19, 2019

Use this procedure to make small fiberglass repairs to cars, boats, or other fiberglass objects. The procedure is especially useful for marine applications. The procedure covers basic repairs, not cosmetic repairs, and does not include instructions for applying gel coat.

Steps

1
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.

2
Note: Resin cures best above 18 degrees Celsius (65 fahrenheit) and in moderate humidity.

3
Note: Polyester resin is porous and may not be suitable for use in areas which will be frequently underwater.

4
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.
5
Clear loose pieces from the damaged area and clean the area with acetone.
6
Tape off the area to be repaired with masking tape.
7
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.
8
Caution: avoid contact with skin, use eye and breathing protection.
9
If using fiberglass filler, mix the filler into the resin until the consistency is like peanut butter.
10
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.
11
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.
12
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.)
13
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.
14
Allow time for the repair to cure based on the recommendations from the resin manufacturer.
15
Caution: Curing composites are hot! DO NOT TOUCH curing composites.
16
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.
17
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.
Community Q&A
Question
Can I use fiberglass for waterproofing?
Community Answer
Yes you can, but don’t forget that the fiberglass layer has a certain thickness. It will make the item you are waterproofing a bit larger.
Question
Can I make a seat podium with fiberglass?
Community Answer
Yes, but it does not have properties suitable to local loading (screws, pitch points, corners), so it would be better to use metal. Stainless steel is most commonly used and likely just as cost effective.
Question
How do I repair hairline cracks in my fiberglass model racing boat?
Community Answer
Use epoxy and a sealer. The epoxy goes in the cracks, and will act as a sort of glue (as epoxy tends to do), and then you can sand it down to shape. The sanded epoxy should be fairly smooth, but when you add water repellent paint over the top, it will be invisible, and your boat will work better than ever.
Question
Can I put fiberglass in and reweld a whole area?
Community Answer
Yes. There is little you cannot repair, however you will likely need new structure, such as wood or foam core, to create a strong repair.
Question
I have a fiberglass water line with a fitting that goes into it. The fiberglass is cracked and the fitting leaks. What do I do?
Community Answer
Fibreglass is a poor material for fluid transfer due to its matrix structure. Depending on the application, I suggest installing a line made from material suitable for the situation, e.g. a rubber hose for cooling water, plastic hose for drinking water, PVC pipe for household waste water, or copper pipe for pressurized water.
Question
How do I prepare a wood hull before fiberglassing?
Community Answer
Simply ensure the wood is dry, structurally strong (rot-free) and bare. A wipe with methylated spirits before fibreglassing will ensure it is clean of oil and grime.
Tips
Don’t buy more resin and hardener than you need. These supplies do not keep well after you open them. Single repair packets can be bought at marine supply stores under the WEST brand. Hardware stores normally carry several resins and, possibly, filler and fiberglass cloth. BONDO is a cheap and widely carried polyester resin.
Warnings
DO NOT PEEL resin from your skin. DO USE a waterless hand cleaner to cleanse resin from your skin.
DO NOT TOUCH curing composites. Curing composites are hot!
Caution: Epoxy resin, polyester resin, and hardening agents are toxic chemicals.
You MUST WEAR proper skin protection, eye protection, and a breathing apparatus.
Things You’ll Need
Rubber gloves or appropriate skin protection
Safety glasses and a breathing apparatus rated for use with epoxy/polyester resins
Rubbing alcohol
Rags
Epoxy or polyester resin, whichever is appropriate
Hardening agent
A mixing cup and stirring utensil
Fiberglass filler or woven roving, if necessary
Tape
Note: Masking tape is best as it is easily applied and removed without residue.
Sandpaper – rough (40-80) grit, medium(100-200) grit, and fine (300+) grit are needed
Rubbing compound and polish to obtain the finest finish (optional)
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About This Article
wikiHow is a “wiki,” similar to Wikipedia, which means that many of our articles are co-written by multiple authors. To create this article, 17 people, some anonymous, worked to edit and improve it over time.
Co-authors: 17
Updated: August 19, 2019
Views: 119,635
Article Rating: 93% – 194 votes
Categories: Boat Service and Repair | Cars & Other Vehicles
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James Shores

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