Fixing Fiberglass Canoes and Boatsby Cliff Jacobson

photo-PFBC file photo


Many years ago, I watched what appeared to be a new fiberglass canoe scrape its way down a rapid I portaged around. I hailed the man and asked him how he could subject a beautiful canoe to such abuse.

"I just hope I live long enough to wear it out," he grinned.

Later that summer I saw the man at a canoe event. His bright-green canoe looked like new. He said he learned how to repair it by attending a seminar sponsored by the U.S. Canoe Association. I joined the organization that day and soon became a canoe fanatic just like him.

You don't need special skills to fix fiberglass canoes. It's easy work and there are lots of second chances if you mess up. Once you get the hang of it, you can usually mend minor gel-coat damage in an hour and major breaks in half a day. And what works for canoes works for any boat. For example, in 1977, I bought a 14-foot sailboat, trailer and all, for $700. The previous owner had run over a wing dam and seriously holed the hull. A marina quoted $300 for repairs. I bought some fiberglass and resin and spent a sunny Saturday mending the tears. Three years later I sold the boat for $900.

Moored boatsField repairs

Generally, it's best to stick with duct tape for field repairs of any boat and save serious patching for the controlled conditions at home. Fiberglassing along a river bank without the proper tools-like sharp scissors and power sander-may later complicate a proper job.

Permanent repairs

First, some terms:

Fiberglass cloth is composed of twisted strands of fiberglass that are woven at right angles to one another. Cloth has the highest glass-to-resin ratio (about 1:1) of all fiberglass materials, and also the greatest strength. Six- or 7.5-ounce-per-square-yard cloth is the most practical weight for mending canoes.

E-glass and S-glass. E-glass is the common boat-building fabric and the one you should use for most canoe repairs. S-glass-a patterned material-is much more abrasion-resistant and expensive.

Episizetm biaxial tape: Two layers of 15-ounce per square yard non-woven E-glass are lightly stitched together at 45 degrees. Biaxial tape comes in 4-inch-wide, 10- and 20-yard rolls. The material is bound at the edges so it's easy to apply along a keel line. For invisible repairs, cover biaxial tape with one layer of 6-ounce E-glass.

Kevlar cloth. This incredibly strong, very expensive "aramid" is used in police flak vests and as a tire cord fiber. All the best fiberglass canoes use some Kevlar to reinforce delicate areas. Kevlar is much stronger and more abrasion- resistant than fiberglass cloth.

Mat: Chopped, cross-linked glass fibers that are held together with a dried resin binder. Glass-to-resin ratio is about 1:3, which makes mat one-third as strong as cloth. Mat becomes very stiff as it absorbs resin, so it's often used in canoe bilges to improve rigidity. Many layers of fiberglass cloth will stiffen an area as well as mat. And the resulting repair will be lighter and stronger.

Gel-coat is a micro-thin waterproof resin on the outside of most fiberglass and Kevlar canoes. It resists light abrasion, but it breaks when the canoe hits rocks. You can replace broken gel-coat, but not without a fight. Gel-coat is difficult to apply and almost impossible to obtain a perfect color match to most boats. The section on mending gel-coat shows how to make cosmetic repairs without using gel-coat.

Colloidal silica is used to thicken epoxy resin. I use it to fill deep cuts and gouges. Colloidal silica is very strong and it sands easily.

Resins: There are polyester, vinylester, and epoxy resins. Epoxy is the strongest and best for repairing canoes. Special boat-building epoxies like Ad-Techtm, System Threetm, and West Systemtm are worth their high price of over $80 a gallon. They come with accurate measuring pumps so you won't waste resin by pouring more than you need.

MEKP. Methyl-ethyl-ketone-peroxide. The hardening agent used for polyester resin. Comes in small plastic tubes and smells awful. Dangerous stuff-it'll blind you if you get it in your eyes! You don't need MEKP if you use epoxy.

Essential tools

Very sharp, long-bladed scissors, orbital and rotary power-sanders, saber saw, cheap paint brushes and plastic squeegees, like those used for body work on cars. You'll also need clean tin cans, paper cups, and glass or polyethylene containers for mixing resins. Never mix resins in a container that holds uncured resin!


Goggles, gloves, and dust masks: Laboratory safety goggles, plastic gloves and dust masks are essential. You'll use lots of gloves and masks, so you might as well buy 'em by the box. Be sure to take all appropriate safety measures.

Always mix and apply resins in a well-ventilated (outside!) area, and wear a good dust mask when you sand fiberglass. Use a new mask for each sanding project.

Protective skin cream (barrier cream): Helps protect your skin from contamination by resin or solvents that get through gloves.

Solvents: You'll need special solvents to remove resin from tools. Acetone is the standard for polyester; epoxy thinner for epoxy. Always wear barrier cream and gloves if you use these dangerous solvents! Some waterless hand creams will remove some resins from skin.

Household vinegar is fine for cleaning tools, but not skin! Vinegar dissolves epoxy just enough to allow it to go deep inside the subdermal tissues of your skin. This increases the chance of an allergic action-and who knows what later on?

In your hair and on your skin! Washing with soap and water seldom removes the tiny fiberglass fibers that cause your skin to itch. In fact, washing may drive the glass particles deeper! Try using a vacuum cleaner to swoop up abrasive dust. If discomfort persists, apply masking tape to the affected area and then gently pull the tape off-like you're removing lint from clothes. This usually works. You may also wash the affected area with a good hair conditioner. The conditioner seems to lubricate and smooth the fibers. Rinse your skin thoroughly after you've applied the conditioner, as you would for hair.


Shallow scratches can often be polished out with auto buffing compounds. Use a light touch on the buffer-bear down hard and you may cut into the glass or Kevlar below!

Deep scratches are best left alone, unless they are so deep that they abrade the glass or Kevlar beneath. If fiberglass or gold furry stuff (damaged Kevlar) protrudes from the scratch, you'll want to make repairs.

If the damage is light, simply flow epoxy resin into the cut. When it has hardened, polish it out and paint it.

Heavily scored or broken Kevlar should be covered with a fiberglass patch. Fair the edges of the hardened patch into the surrounding hull, and then paint it. Use fiberglass for cosmetic patches. Kevlar cannot be sanded!

Mending gel-coat

One trip down a rocky river and you'll need to repair the gel-coat on the nose of your canoe. Gel-coat repair kits do a good job if you're very patient. The book procedure calls for filling the break with color-matched liquid gel-coat (which is runny and hard to contain), and then sanding and polishing to blend the repair.

This is slow, frustrating work, and it's almost impossible to get a perfect color match.

This procedure is easier and faster, and the finished repair is invisible.



1. Pick out the shards of damaged gel coat.

2. Catalyze the polyester putty (use extra MEKP for a hot mix) and work it into the break to overflowing. If you're using epoxy, stir in colloidal silica until you get a peanut-butter-thick mix that won't run.

3. When the resin has cured, sand it level. Finish to silky smoothness with 400 grit wet sandpaper.

4. Paint the patch. Later, use a mixture of paste wax and pumice to blend the paint to the hull.

If you have a natural gold Kevlar canoe, or one whose color you can't match, repair the gel-coat as described above-then mask a short, artificial waterline along the bow and stern (see Figure 1). Paint the masked area an attractive color. The paint will hide your repair. 

Figure 1

graphic-Ted Walke

Skid plates

Some paddlers glue thick Kevlar "skid plates" on the ends of their canoe to protect them from damage. Here's why this may not be a very good idea:

1. Kevlar skid pads may absorb a pint or more of epoxy resin, which increases the weight of the canoe by several pounds. This weight is added at the extreme ends of the canoe, which affects its "swing weight," or ability to carve and check turns. If you want to turn a high-performance canoe into a barking dog, just add some weight to its ends. You'll notice the flywheel effect immediately.

2. You can't sand Kevlar, so the harsh edges of the thick felt pads won't fair in to match the contours of the hull. The result is increased wetted surface and noise.

Here's a better plan: Lightly sand the area to be patched and cover it with one layer of biaxial tape. Saturate the fabric with resin (a plastic squeegee works better than a brush). Then lay an oversized "cover piece" of 6-ounce E-glass or S-glass on top. Work resin into the material until the "cover patch" is saturated. When the resin is dry, fair the edges and lightly sand the surface of the fiberglass until it is smooth to the touch. Spray paint to match the hull.

Two essential patches

These two patches can repair most any boat:

1. Double-sided patch. The damaged area is accessible from both sides of the hull.

Tip: You can increase the strength of your repair by substituting one or more layers of Kevlar for fiberglass. Remember, Kevlar can't be sanded, so always use a fiberglass "cover patch."

 Figure 2  Figure 2b
graphic-Ted Walke 

2. Single-sided patch. The damaged area is accessible from one side only. Use this method when flotation foam prevents you from getting to the break.

Tip: Fiberglass patches will adhere better to tightly curved areas if you cut them diagonally across the weave, as illustrated in Figure 4.

 Figure 3

 Figure 4

graphic-Ted Walke

I am indebted to Bob Brown, past building chairman for the Minnesota Canoe Association, for these unique ways to repair fiberglass/Kevlar canoes. Bob has designed and built dozens of canoes, kayaks, and sailboats.

Hull repair

Remove the chards of broken gel-coat.

Hull repair

Catalyze the polyester resin and apply it to the break. If you use epoxy, thicken it with colloidal silica to a peanut butterlike thickness.

 Hull repair

Sand the hardened resin flush with the 

surrounding gel-coat.

Hull repair

Paint, polish and wax.


sequence photos-Cliff Jacobson

July/August 1999 Angler & Boater

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