About Fiberglass Part 2: Gelcoat repairs February 19, 2018 02:26

Note: This article deals with repairs above the waterline (for water-induced blisters, see Part 3). It does NOT cover repair or replacement of non-skid deck, since this is a separate and more complex subject. This article was originally prepared for a forum I hosted on Sailboatowners.com.

Background: Gelcoat is much softer than polyurethane paint, but is present in thicker layers that allow more depth for sanding and compounding. It is much cheaper and easier to apply than polyurethane. However, gelcoat is less durable than polyurethane paint and more easily suffers from nicks, scratches, stress cracks and abrasion. It also oxidizes more rapidly.

Another, more rare problem is lack of adhesion of the gelcoat layer to the original fiberglass laminate. This is possibly due to delay is applying the original fiberglass laminates to the layer of gelcoat lining the mold. This can result in loss of dime-sized pieces of gelcoat. Another form of damage is wearing through of the gelcoat  layer by abrasion. This can also happen with excessive wet sanding and compounding. In an attempt to remove oxidation that is deep-seated.  It can also be caused by the boat rubbing against a piling during a storm. The final type of gelcoat damage is the formation of “alligator-skin” type cracks and/or “crazing” caused by severe and repeated sun exposure.

How are such damaged areas repaired? Similarities and differences in procedure are described below. We will start with a description of items needed before beginning the repairs.

Required Tools etc

For opening up nicks and scratches before repair you will need a scraper or sharp-edged screw driver. You will also need sandpaper, ranging from coarse (80 or 100 grit) to 220-600 grit “wet or dry” silicon-carbide paper. Small amounts of coarse and fine rubbing compound (I like Turtle Wax® brand) and a fine polish like those from Meguires®. Some wax for final touch up is also required (we like Collinite Fleetwax 885) or our own SIMPLY BRILLIANT Superwax™ and/or MIRROR HARD Superglaze™ resin coatings. A Dremel® high speed grinder is useful and also  an electric buffer for compounding and polishing.

Graduated polyethylene vessels are essential and some mixing sticks (coffee-stirring sticks can also be used for small batches of gelcoat.). Some Mylar sheet  or polyethylene (plastic bag material) are needed for placing over uncured repairs to aid shape and/or keep air from the surface while the resin is curing. Plastic squeegee applicators are required for spreading on filler or thickened gelcoat. If gelcoat is going to be sprayed on, use propellant-driven disposable spray guns. The one made by Preval® is most suitable for small repairs. Before spraying, the gelcoat will thinned and filtered to remove lumps and particles that might clog the spray-head.

Masking tape and/or ISLAND GIRL®s “Hawaiian Blue Masking Gel™ are needed to mask the surrounding areas. The masking gel can even be used to mask textured non-skid decks and vinyl upholstery where masking tape cannot. “Fine-line tape" from 3M will be required if the edge of a stripe is involved in the repair. Finally, a paper dust mask is recommended during sanding gelcoat and filler. A charcoal-filtered mask and safety glasses are necessary during gelcoat spraying.  You should wear thick-walled rubber kitchen gloves as well as old clothing or coveralls.


Polyester resin (clear, non-tinted) is the basis of all gelcoat repairs. When it is tinted with colored pigment or dye it is termed “gelcoat”. Regular polyester (laminating) resin is “air inhibited” so it sets with a sticky surface unless the surface is sealed from air. This can be accomplished by pressing plastic film over the repair or by spraying with polyvinyl alcohol (PVA) surface sealant. PVA is also a mold release agent so it can be used under the plastic film to make it easier to peel off the cured resin.  Cured PVA is removed by water, so you will also need a water hose or other source of tap water.

Wax/styrene “surfacing agent “ can be added to regular resin, converting it into “surfacing resin” which can also be purchased in ready-mixed form. We prefer using this type of resin because the wax rises to surface, sealing it so that no surface coating (plastic film and/or PVA) is required to prevent surface stickiness.  However, because wax is partially imbedded in the surface, it cannot be removed  by solvents, but wet sanding is required prior to painting, or making further gelcoat repairs..

Precaution: Set-up times must be sufficient (more than 10 minutes) for the wax to rise to the surface evenly. If more than one layer of catalyzed surfacing resin is applied, the surface of the underlying layer must  still tacky, otherwise sanding between layers is necessary.

For manual (rather than spray) application to deep scratches, or on vertical surfaces, the gelcoat has to be thickened so that it does not run or sag. This can be done by the addition of Cab-O-Sil® resin powder. This renders the resin mix to be “thixotropic” i.e. it behaves both like a liquid when stirred, but gelatinous after application. The thickening powder does not alter the effects of added color pigments. Most small “do-it your-self” gelcoat repair kits contain surfacing resin that is already thickened with Cab-O-Sil®.

For making up a more “Bondo®-like type of paste for filling deeper repairs, talc (a magnesium silicate) is the cheapest (and heaviest) filling agent for the resin. Plastic micro-balloons are lighter and make a paste that is more easily sanded after curing. Glass beads or other harder-to-sand fillers are not recommended for gelcoat repairs. You can also use Bondo® or other body fillers, available in kits, some of which are especially formulated for marine use.

The resin becomes “gelcoat” when it is colored by application of pigments (powders of solid color) or dyes (soluble in the resin. Dyes are seldom used except on surfboards and/or metalflake finishes.

Thinners for Spraying Gelcoat

When spraying gelcoat, you can use either regular resin or surfacing resin, no thickening with Cab-O-Sil® is allowed. Instead, the resin is diluted down with styrene and acetone (no more than 10% acetone). Too much acetone dulls the surface but styrene is much slower to flash off. Some people like to use a “slow” lacquer thinner with the styrene instead of acetone. Thinning between 15 and  50% may be necessary, depending upon spray gun, operator, ambient temperature etc. Some trial and error practice is required.

Dewaxing procedure, prior to application.

First scrub the area with a strong cleaner such as Ajax® or Superclean® . Then use a Xylene-based “grease and wax remove”, that will also remove all silicone residues. It is available at any Auto paint store. . For small jobs you can use “Goof Off”. Followed by acetone or lacquer thinner. DO NOT WIPE ON WITH SHOP RAGS because the silicones used in fabric softener may affect adhesion. Use only Cheap unprinted paper towel.


The polyester gelcoat or paste filler (if used) is induced to set hard by mixing with the proper amount of catalyst according to instructions.  For paste filler repairs (not gelcoat) a bondo-type catalyst can be used which has a dye to assist in thorough mixing.  For gelcoat, however, colorless catalyst is used, applied with a drop bottle. It is important to mix very thoroughly without adding excessive amounts of catalyst that may either interfere with proper pot life or hardness of the cured resin. Usually only the equivalent  of 1-4 Oz Catalyst per gallon is used depending upon temperature (less catalyst on a hot day). Finally, avoid temperatures below 50 degrees F (resin may not cure properly) or abuve 90 degrees (sets up too fast). Also avoid working in direct sunlight except for very small repairs since UV light accelerates hardening and solvent bubbles may ruin the finish.

Color Matching of Gelcoat is dealt with in detail HERE

It is generally easier to color match liquid gelcoat rather than thickened paste. When diluted with thinner for spraying there will be an apparent change in color, but the original gelcoat color match is still there. When Cab-O-Sil® is used to thicken previously color-matched gelcoat, the original color match is not affected. Color-matching with gelcoat paste can be also be done.

Repairs of Nicks and scratches

This type of gelcoat damage is caused by collision with a sharp object or collision with something heavy and sharp. Such damage is fairly easy to repair.

Step 1:

After dewaxing, the local area and light sanding (600 wet or dry) to expose fresh original gelcoat, the scratches or nicks are opened up by the edge of a scraper and rough sanded (80grit). The opened up area is wiped out with some acetone, to remove debris and then a little styrene to aid chemical bonding of the repair.

Step 2:

We advise use of a surfacing resin for making up the gelcoat repair mix. Color-matched gelcoat (i.e. polyester resin, plus pigment) is suitably thickened with Cab-O-Sil®, then catalyzed and applied with a plastic applicator to the area, filling the entire area  proud.  On clear (or tinted) gelcoat over metal-flake, no pigment or perhaps a little color-matched dye is used.

Step 3:

The repaired area is sanded smooth with 220 grit, then (stepwise) down to 600 wet or dry sandpaper. The area is further smoothed with rubbing compound, then polished and waxed.

Note: if the original scratch is so deep that it reaches down or into the underlying fiberglass laminate, some repairmen prefer to use an under-layer of polyester filler paste and apply the outer layer of gelcoat by spray, as described for repair of stress cracks (see below).

Stress cracks

The polyester resins of the gelcoat slowly become more brittle with age and are therefore more prone to stress cracks. They tend to form around metal fittings due to alternate heat expansion and contraction. They also form (as on my boat) in some areas where the boat hit the dock and the fiberglass hull flexed. Because the gelcoat in thick and has no fibers for reinforcement, it is less flexible and may form some cracks that go down to the underlying fiberglass laminate, with some lack of adhesion between the laminate and the gelcoat. If cracks are due to underlying and persistent structural weakness causing flexure, this weakness should be fixed before the gelcoat is repaired.  Stress cracks are  repaired in a different way than for nicks and scratches.

Minor Stress Cracks

It is important to prevent the cracks from spreading by preventing water from getting into the cracks. In cold climates.  The expansion produced by freezing water may  enlarge  the cracks and even start to lift the gelcoat layer from the underlying laminate.

One way to fix these minor cracks is to scrape out any dirt with a fine needle and then fill the cracks with (catalyzed) penetrating epoxy resin. This will seal the cracks and ensure that adherence of the gelcoat is ensured.

In milder climatic conditions, we have found that minor cracks can be filled by MIRROR HARD Superglaze™. Indeed we have testimonials from a customer who has used our 2-Step system to ameliorate surface crazing of the gelcoat surface, shown HERE.

More Serious Cracks

These need to be opened up and filled as described below:

Step 1

De-wax and then wet sand (220-400 grit) the area to expose true colo of the gelcoat, prior to use of our NEUTRAL CLEAR™ Cleanser-Conditioner often assists in brightening up porous gelcoat to its true color.

Step 2:

Grind out the area (using a Dremel® tool) until it is obvious that the area has been reached where there is solid bonding between gelcoat and underlying laminate.

Step 3.

Wipe out the area with acetone to remove debris, then with some styrene to increase chemical bonding of the repair.

Step 4:

Fill the area to just above the original (sanded) gelcoat surface  with catalysed paste filler. The final intention is to cover the area with sprayed-on color-matched gelcoat.

You can buy a ready prepared Bondo®-type body filler or (better) you can make your own using regular resin mixed with Cab-O-Sil® (to prevent runs) with talc or plastic micro-balloon filler. Pigments may be added to approximate color of gelcoat in an aid to hiding: the color of the filler underneath the outer sprayed gelcoat layers.

Step 5:

Sand the area to original color, using small amount of additional filler to fill in any low spots. The final step is to blend in the repair as much as possible, by sanding with 220 grit wet- sanding.


Then wipe of the repair area with acetone or lacquer thinner and mask off the surrounding area with masking tape and/or Hawaiian Blue Masking Gel™.  “Back taping”: (peeling back edge of the tape) can be used to provide a soft edge” to the sprayed gelcoat. Another way of making the repair less detectable is to mask to a “factory edge” where minor differences in gelcoat shade will not be noticed.  Be especaially careful to protect textured non-skid decking, upholstery and plastic windows where overspray damage can be serious (our Hawaiian Blue Masking Gel™ is ideal for this). IG Pink™ followed by water rinsing can remove uncured overspray or spills on delicate surfaces. DO NOT USE ACETONE OR LACQUER THINNER THAT WILL DAMAGE THE UNDERLYING SURFACE


Spray with color-matched, thinned gelcoat using the disposable sprayer. Remember pot life of catalyzed gelcoat is only about 15 min before it becomes a gel (rinse gun immediately) with acetone or lacquer thinner. Apply several coats. Unlike using paint (which would run), one coat of gelcoat can immediately follow the previous one. Even if runs do occur, they are easily sanded out at the end.

 Step 7A:

This “blending process can continue to essentially clear gelcoat if you are a perfectionist.

Although not absolutely necessary, if surfacing resin was used to make the gelcoat (it dries tack-free), final spray application of PVA will a ensure  shiny surface. If regular resin was used to make the galcoat,  spraying of a least 3 coats of PVA during final cure of the surface is absolutely necessary.

Step 8:

When the gelcoat has hardened sufficiently, remove the tape by pulling “backwards” on the tape and not straight up. Be especially careful if the tape was used at the edge of a stripe. Not less that 24 hours later, lightly wet sand with 600-800 grit, then compound, polish and wax the area to make it virtually undetectable.

Repairing Abrasion Damage

If abrasion damage is widespread, a complete paint job or (cheaper) re-gelcoating will be needed. However, localized abrasion caused by careless use of a buffer, or rubbing against a piling, can be repaired in a cost-effective way.  The damaged surface is de-waxed and sanded as appropriate. Filler is only necessary to bring up to level of the underlying laminate. If abrasion has damaged the laminate, then a laminating repair may have to be done first, using resin and fiberglass matting (beyond the scope of this article). Otherwise, follow the steps described above for repair of stress cracks.

Repairing gelcoat adhesion damage (loss of pieces of gelcoat)

The problem here is ascertaining the extent of the problem. Obviously, if more of the gelcoat is not properly bonded to the underlying laminate, the only real cure is to remove ALL of the gelcoat and resurface with new gelcoat or else polyurethane, over a catalyxed epoxy primer.

However, we will assume here that the problem is localized to those areas where gelcoat has fallen off in small pieces. Then the repair is done basically as described for serious stress cracks.

Repair or “alligatoring” and crazing

Short of removing all the gelcoat and repainting or re-gelcoating, the easiest way to fix such damage is to de-wax and sand down to a reasonable level of smoothness. Then, after masking, apply one or three coats of 2-part (catalyzed) epoxy primer paint with wet-sanding  (220 grit) between coats. Addition of a little “Fish-Eye Eliminator” (actually a silicone itself) to the paint is a good idea if you think that traces of wax-silicone may remain on the surface. The excellent bonding and flexibility characteristics of the primer will fill in the cracks. After wet-sanding, the final coat of primer to to suitable smoothness (400 grit) apply one to three coats of two-part polyurethane such as Interlux®  by brush, roller or spray with sanding between coats, unless the finish is already smooth (but still soft) when a final coat can be applied.

without all this work, we have at least one report of the ISLAND GIRL® System ameliorating the effects of gelcoat crazing

Materials for these repairs are available from West Marine, TAP Plastics and Fiberglass Hawaii. - You can look them on on the Internet!