This article was originally published in December 2000 on SailNet.
Through SailNet’s "Ask the Expert's" program, a significant number of users have requested help in rejuvenating or repairing the gelcoat on their boats. Problems cited include chalking, fading, crazing, chips, and cracks. In hopes of answering all of those questions at once, I thought I would provide a short course on gelcoat.
What is Gelcoat? The initial laminate in the layered construction of a production fiberglass boat is something we call gelcoat. Sprayed into a highly polished mold, it gives the boat both color and gloss. The structural layers of fiberglass are applied to the "back" side of the gelcoat. This is opposite of the way most other products are manufactured, where the last step in production is to spray on the finish—presumably the reason it’s called the finish. When it comes to production-built boats, gelcoat is the starting point.
Gelcoat differs from paint in other important ways. Paint bonds to the underlying surface mechanically—by gripping microscopic scratches put there by sanding or etching. The bond between gelcoat and the underlying laminates is molecular. Resin saturating the first layer of glass material combines with the exposed surface of the gelcoat to form a single mass—not unlike pouring warm gelatin over cold. This is called chemical cross-linking, and it occurs because the gelcoat resin and the polyester resin used to saturate the layers of fiberglass fabric are the same basic product. Gelcoat is essentially pigmented polyester resin.
Gelcoat does not "flow out" like paint. Good paints are self-leveling—like water. They dry to a smooth, glossy finish. Gelcoat behaves more like plaster, taking on the texture of the application tool. It can be thinned and sprayed to get a reasonably smooth finish, but the wet-look gloss characteristic of new fiberglass boats is due entirely to the polished interior surface of the mold.
Gelcoat is also about 10 times as thick as a paint finish. This is both a blessing and a curse, as we will see.
Chalking Well-applied gelcoat (like everything else, there are quality differences between manufacturers) can look good for a decade with minimal care. An annual coat of wax doubles the gloss life, but the elements eventually erode the relatively soft gelcoat.
|"Regardless of the restorative measure, the first step is always a thorough washing."|
The loss of gloss is due to this roughening of the surface. There are two strategies for restoring the gloss. One involves filling the microscopic surface pits; the other requires removing the rough top surface.
Regardless of the restorative measure, the first step is always a thorough washing. However, detergents do not necessarily lift oil and grease from the gelcoat’s pores. Oily contaminants must be removed if you are to get dependable results from wax or polish. Wearing rubber gloves to protect your skin, wipe the gelcoat with a rag saturated with MEK (preferred) or acetone. Turn the rag often and replace it when you run out of clean areas.
Wax Keeping gelcoat coated with wax—starting when the boat is new—is the best way to prolong its life. The purpose of wax is to protect, but it also has restorative properties when the gelcoat is not too badly weathered. The wax fills microscopic pitting in the gelcoat and provides a smooth, reflective surface. The gentlest of all restorative measures, waxing, should be the first thing you try when attempting to restore the luster to your hull.
Polish Polish is an abrasive—like extremely fine sandpaper. When wax fails to restore the shine, polish is the next step. Rather than filling the pits, polish grinds them off, exposing a fresh, smooth surface. Polishing is a mechanical process—you rub the surface with a circular motion until it becomes glassy. An electric buffer is highly recommended for this.For polish to cut evenly, you must first remove all wax from the surface using a dewax solvent or toluene. After polishing, apply a coat of wax to protect the surface and improve the gloss. (Some polish products include wax in their formulations.)
Compound The next weapon in the arsenal is rubbing compound. Compound contains more aggressive abrasives than polish. Select a rubbing compound formulated for fiberglass and use it exactly like polish, rubbing it with a circular motion until the surface turns glassy. Because of the thickness of the gelcoat, compound shouldn’t cut all the way through if you are careful not to rub in one place too long. If the gelcoat starts to look transparent, stop.
After the surface has been compounded, polish it, then coat it with wax and buff it. Providing the gelcoat has adequate thickness—the boat might have been compounded previously—this process will restore the shine to gelcoat in almost any condition.
Sandpaper Occasionally, the dead layer of old gelcoat is so deep that removing it with rubbing compound becomes interminable. In such cases, the process can be accelerated by sanding. This expedient only works if the gelcoat is thick. You can test this by scratching the surface in an inconspicuous spot, but the thickness may not be uniform over the whole boat. Sanding risks cutting through the gelcoat, committing you to painting, so you should attempt this only if painting already seems like your only alternative. If you are successful, you avoid painting; if the gelcoat proves too thin, you are no worse off, and you have done a significant amount of the prep for painting.
If you decide to take the risk, use a palm sander loaded with 120-grit aluminum-oxide paper (it’s brown). Apply only as much pressure as needed to maintain contact and keep the sander moving. It is working at about 200 orbits per second, so don’t sand any area more than a few seconds. Don’t run the sander over ridges, high spots, or corners, or you will cut through the gelcoat regardless of how thick it is.
A word to the wise: Wear earplugs. They not only protect you from the sander’s ear-damaging shriek, but because they eliminate the fatigue that accompanies such an assault on the senses, they actually make this job much easier.
|"A word to the wise: Wear earplugs. They not only protect you from the sander’s ear-damaging shriek, but because they eliminate the fatigue that accompanies such an assault on the senses, they actually make this job much easier."|
Because the first pass removes most of the material, if the gelcoat doesn’t get transparent, good results from the remaining steps are likely. Reload your machine with 220-grit paper and sand again. Then wet-sand by hand—first with 400-grit wet-or-dry (silicone carbide) sandpaper, then with 600-grit. Finish the job by buffing the surface with compound and/or polish, followed by a protective coat of wax.
Restorer A number of products have come on the market lately that claim to "restore" the surface of the gelcoat. Restorer formulations renew the gloss in essentially the same way as wax—by filling the pits to provide a new, smooth surface. The results can be dramatic, but because restorers are a plastic (typically acrylic) coating—similar to urethane varnish—they can wear off, flake off, and occasionally discolor.
Restorer kits typically include a prep wash and sometimes a polish in addition to the restorer. A specialized stripper—for removing old sealer—is also necessary. There are variations in the recommended application, but in general it is clean, polish, and then coat. The acrylic sealer is usually water-thin, so it is easier to apply than paste wax. It dries to a hard film so no buffing is needed, but you do have to apply several coats to get a good shine. Drying times are short, so subsequent coats can generally be applied almost immediately. A full application should last for a year, or perhaps longer.Fading Colored gelcoats can fade with prolonged exposure. This loss of intensity can be due to surface erosion or ultraviolet bleaching. In the first instance, all the measures outlined above dealing with chalking are equally effective at color restoration. If, however, the pigment has been damaged by the sun, coating the surface will not help. To restore the color you must expose undamaged gelcoat. You can check this by simply wetting the hull. If the color returns to full intensity, a coating treatment should be effective. If the color remains washed out, only an abrasive treatment has any chance of success.
Paint When all else fails, any gelcoat surface can be restored with paint. A two-part polyurethane paint can make old gelcoat look better than new, but this is a subject for a different column.
For crazing, chipping, and cracking, meet me here again next month and I will try to provide some useful hints. Meanwhile, for a more complete and fully illustrated explanation of gelcoat repair, I recommend Sailboat Hull & Deck Repair—for sale in SailNet's Store or available at your local library.
Gelcoat Repair by Tom Wood
New Shine for an Old Hull by Don Casey
Old Hull, New Gelcoat by Tom Wood
SailNet Store Section: Maintenance