How to Repair Fiberglass

Fix boats, personal watercraft, even truck toppers. Do it yourself and save thousands.

A repair pro shows you how to make invisible fixes for scuffs, dents and scratches in fiberglass boats and other fiberglass items. You'll save a ton of money if you're patient and use the proper tools.

By the DIY experts of The Family Handyman Magazine


One day




$100 - $500

Overview: Cost, savings and repair strategy

Even the best boat captain has had a docking turn into a scuff-and-chip event. But you don't have to pay astronomical marina fees to get your boat fixed. You can do it yourself with advice from expert fiberglass repair specialist Chris Hassis. He's fixed boo-boos on fiberglass personal watercraft, snowmobiles and pickup toppers, and even luxury yachts. You'll need the right tools and materials—and lots of patience. But he'll show you how to save a ton of money and make your boat look like new. We were astounded at this refurbished boat's rebirth. The boat now looks every bit as good as it did in the showroom.

Most hired-out repairs of any type are roughly 50 percent labor and 50 percent materials. Not so with this fix—hull repairs are all about labor. The repairs on this boat would cost roughly $3,000 at a shop. But the materials only cost about $250 and the buffer about $150! So roll up your sleeves. Your repairs might not be perfect, but your boat will look unbelievably better than it did before. Just don't try to rush or skimp on the materials. And know that this job is much bigger than it looks.

All fiberglass boats have a thick structural core of fiberglass strands impregnated with polyester resin. But the part you actually see is the gel coat. That's the thin layer of pigmented resin that gives the boat its sheen and color. Most light scratches are in the gel coat layer. They can be sanded out and the area built back up with new gel coat. But if they penetrate the core, they need to be filled. You can tell that the scratch is into the core if you see fiberglass fibers (Photo 2).

Remove graphics

No boat spiff-up is complete without new license numbers, transom names and pinstripes. Just do an online search for “boat graphics” and you'll find thousands of options to choose from. Find stripes by searching for “boat pinstripes.” Remove the old ones by warming them with a heat gun (Photo 1). But be careful—if you overheat the surface, you can burn the gel coat. If yours are painted on, sand them off with 1,200-grit sandpaper.

Fix gouges: Prep

Grind out gouged areas with a V-shaped grinder bit (Champion SF1; from; Photo 2). Sand out the light scratches, starting with 80-grit and ending with 240-grit.

Fix gouges: Fill and smooth

To fix gouges and deep scratches, you'll need “chop” (powdered fiberglass) filler, gel coat and gel coat reducer. Jot down your boat's model and serial numbers and contact the manufacturer to order gel coat (about $100 per qt.) to match your boat's color. It may not be an exact match, but it'll be a lot closer than mixing colors from scratch. Then order some gel coat reducer (Patch-Aid is one brand; about $49 per qt. from or to thin the mixture enough to go through the spray gun. For gouge repair, you'll mix chop filler (Chris uses Cab-O-Sil from with the gel coat to create a thick paste. Then pick up paper cups, stir sticks, acetone (for cleanup) and rags. You'll also need a disposable spray gun (Preval is one brand; about $7 online or at paint and art supply stores). To finish the job, you'll need a professional-type buffer. The variable-speed feature is important, and so is the high power of a professional buffer. So don't skimp on one or think you can get by with a $29 wax polisher. Use a wool buffing pad (such as a Dewalt DW4988 pad, available through our affiliation with or, and buffing compound (3M Imperial Compound and Finishing Material, which is what Chris uses, available through our affiliation with or

Mixing gel coat is a messy and stinky process—so wear chemical-resistant gloves, safety goggles and a respirator. Start with a small amount of gel coat and stir in the chop and hardener to make a putty that's the consistency of peanut butter (Photo 3). Once you add the hardener, you've only got a 10- to 20-minute “open” time, so mix small batches and work on one gouge at a time. Thorough mixing is critical to proper curing. Spend a full minute stirring. If you don't, you'll end up with patches of sticky resin that can take days to harden.

Overfill gouges so the filler mounds slightly. You'll sand it flush after it cures. Curing can take one to two hours—depending on humidity levels. So test it by touch. If it's sticky, it's not fully cured. Once it's fully hardened, sand it with 80-grit sandpaper (Photo 5).

Color-Matching Gel Coats

Sometimes you can't get your hands on factory gel coat (your boat is too old or the manufacturer is out of business). Then you have two options. You can order an off-the-shelf color that's close to yours and decide that “close enough is good enough.” The other option is to custom mix, but be aware that this is no easy task. Buy a color chip chart for off-the-shelf colors (from a local marine supplier, or get the No. 01900 color chart from for about $12). Mix your own color with a kit (No. 33114; about $35) from

Fix gouges: Apply the finish

Now you're ready to mix a fresh batch of gel coat (without the chop) and spray the scratched and patched areas (Photos 6 and 7). Chris recommends using a disposable spray gun and paper cups. Unthinned gel coat won't go through the sprayer. Add the recommended amount of reducer (read label directions) and hardener and mix the ingredients. Spray the patched areas with short bursts. Spraying gel coat isn't like spraying paint. It splatters on and has to be sanded and buffed to get to a smooth gloss. So don't be disappointed that the finish isn't paint-like right away.

Final-sand, buff and wax

Wait for the gel coat to cure. Sand the repairs with 600-grit and then 800-grit sandpaper. For the perfect finish, sand with 1,200-grit wet/ dry paper then buff (Photo 8). Apply buffing compound directly to the pad. Work on a 2 x 2-ft. area and use light to medium pressure at a fairly low speed. Reduce pressure as the compound starts to dry. Wipe off the haze as you go. Apply the graphics and then wax.

Order of Events

Wash and rinse the boat. That will show you all the dings that need fixing. Mark them with masking tape so you won't forget any.

If you're replacing pinstripes, license decals or other graphics, remove them next.

Fill any deep gouges and spend your time sanding out scratches while the filler sets up.

Apply gel coat to the filled gouges and finish to the scratches and then final-sand the filled gouges.

Buff all the repaired areas and then the rest of the boat.

Apply any graphics to finish up.

Required Tools for this Project

Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.

  • Heat gun
  • Safety glasses
  • Sanding block
  • Rotary tool

You'll also need a variable speed buffer, wool buffing pad, chemical-resistant gloves, respirator and a burr-nose grinder bit.

Required Materials for this Project

Avoid last-minute shopping trips by having all your materials ready ahead of time. Here's a list.

  • Powdered fiberglass filler
  • Gel coat
  • Gel coat reducer
  • Disposable spray gun
  • Acetone
  • Buffing compound
  • Wax
  • Sandpaper, 1200-, 800-, 600-, 240-, 150- and 80-grit

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