The Pro's Guide to Restoring Gelcoat

Posted by Ann Key at 10:42AM - Comments: (2)

March 18, 2014

Nelson Roberts of Atlantis Boatworks brings the red back on a Compaq 20.

We’ve just wrapped up a test of aggressive rubbing compounds for heavily oxidized fiberglass hulls—now available to subscribers in the April 2014 online issue—and this also gave us an opportunity to look at the tools and techniques involved in resuscitating a neglected hull. Although this can be tedious work, it is one of those jobs, like varnishing, that offers instant gratification. The fruits of your labor—unlike that new sanitation hose you sweated over last fall—are apparent to everyone.

Restoring old gelcoat is a matter of degrees. You’ll need to inspect the health of your gelcoat and decide which solution is best for you. If restoring the hull requires wet-sanding or rubbing compound, the general rule is to start with the least aggressive approach. Only when it’s obvious that you are making no progress with an abrasive material (either sandpaper or compound) should you move up to more aggressive media.

If the gelcoat is deeply scratched, gouged, or looking thin in some areas, it may be time to sand and paint the hull (PS, December 2012, February 2011, August 2009). Remember that gelcoat is only a thin layer to begin with—it varies from boat to boat, and it’s thinnest at the bow, corners, and curves—and an aggressive buffing compound may take that layer down to fiberglass. Using a gelcoat restorer (PS, July 2011) is another option, but be aware of their pros and cons.

If oxidation and minor scratches are your problems, a rubbing compound system (followed by a wax) should be enough to restore shine. However, if the rubbing compound is getting you nowhere (try a test patch), and after washing the rubbed area, the oxidation is still visible, it’s time to wet-sand the hull. Assuming, of course, you’ve got enough gelcoat to sand.

Follow the wet sand with a fast-cut, coarse-grit rubbing compound. This can be followed by a medium- or fine-grit compound; if the boat is less than three years old or the oxidation is only mild, you can go straight to a medium- or fine-grit compound. Follow the compounds with a polish and a good boat wax (PS, September 2010 and July 2009) like Collinite’s No. 885 Fleetwax. A good wax will help protect the hull, so you won’t have to buff so aggressively in the future.

The Swedish-made Mirka sander is as compact and powerful as any pneumatic tool.

Buffing Techniques and Tips

Wet sanding: Start with 600- to 800-grit wet-dry sandpaper. Pros we talked with prefer Sunmight (www.sunmightusa.com), 3M (www.3m.com), and Mirka (www.mirka.com) sandpapers; they don’t load up as quickly and last longer. While you can wet-sand by hand using a rubber block, we suggest using a pneumatic or electric, dual-action (DA) orbital sander to make the job faster. Nelson Roberts of Atlantis Boatworks (www.atlantisboatworks.com) in Sarasota, Fla., showed us his coveted sander, a pricey device made by Mirka, which has developed an alternative to pneumatic and heavy electric tools. 3M recommends finish sanding with 1500-grit because it’s easier to buff out a 1500-grit scratch then an 800-grit one. Be sure to keep the hull wet. If you’re sanding by hand, soak the sandpaper in a bucket of fresh water until you’re ready to use it. Be very careful at the hull corners and the top of the topsides where gelcoat is usually the thinnest.

If you’re using an electric sander, mist the hull surface with a spray bottle. Mix a few drops of dish detergent in the water to keep the hull or paper evenly wet and keep it wetter longer. Rinse the surface often to look for potential burn-through areas, and look at it from several angles. You can use a window squeegee to quick dry the surface after a rinse to get a low-glare look at the gelcoat. Do not use circular movements. Wet-sand until the hull has an even dullness, a matte finish; then rinse with fresh water.

Compounds: Apply the compound generously to the buffing pad, keep it wet, and go slowly. If you don’t feel any drag on the pad, and it’s sliding around during application, it’s time to clean or change the pad. Scrape it and add more product—a star tool or a screwdriver can help clean a loaded pad—or change the pad. After applying the compound, wipe off excess with a clean rag; if it’s hard to wipe off, you can wet the area with fresh water, then wipe.

Compared with sandpaper, the Mirka "cheesegrater" abrasive pads last much longer without clogging.

Compounding Tools

A successful refinishing job is the result of using the right products and the right tools. We recommend using a machine polisher for applying rubbing compound. In PS Editor Darrell Nicholson's previous blog post, “Waxing and Polishing Your Boat,” he talked about a few tools, including the one he uses for big jobs: the DeWalt DW849, a Dewalt variable-speed polisher. (By the way, if you haven’t checked out that blog post yet, your probably should. It is the mother-of-all-fiberglass restoring posts, with some great links.)

For the buffing compound test, we took a cue from local pros and used a Shurhold 3100 variable-speed electric polisher. The 3100 is lightweight, has six speeds, an adjustable D handle, and an optional side handle. With the 3100, start with it set on 3 or 4 (3 is roughly 1500 RPM); add speed for a shinier finish. 3M recommends running a buffer at 1500-1800 RPM. The Mirka tool is speed sensitive when buffing, so for the best gloss, start at a slow speed, grind in the compound well, then move to a faster speed to finish. To get to tight, small areas, use an angle-drill with 3-inch pads, which Mirka makes. For a detailed look at other options for buffer/polishers, check out “Waxing and Polishing Your Boat.”

Wool buffing pads are better than foam or cotton because they hold the rubbing compound better. Quality pads can last years. To reuse a buffing pad, wash it in a clothes washing machine using warm water; we suggest doing this at the laundromat. Follow with a load or two of rags to soak up any possible compound residue. Also, Shurhold makes a specialty buffing pad cleaner, the Serious Pad Cleaner, which we reviewed in the April 2014 Chandlery.

Comments (2)

A deck properly prepped and say Awlgriped can certainly look beautiful but it is a difficult finish to maintain under the normal hard use a deck and cockpit can see. I refinished our last boat exactly that way. The boat was a badly abused Able Whistler 32 that had really suffered from about 10 years in the Bahamas. When finished with the deck and hull the boat truly looked new. The problem was that it was really challenging to keep it looking nice. Every time a scratch happened it was a glaring fault that required some expensive touch up.

If I had it to do again I would think about re gel coating instead. I am sure that is not inexpensive either but generally far more forgiving to live with.

Good luck!

Posted by: Bruce B | March 26, 2014 6:29 PM    Report this comment

This is a great article and I've seen many similar over the years directed towards refinishing topside. However, I've seen no discussion about restoring decks.

I have an 18 year old boat and I'm beginning to see pinholes in the deck gelcoat, particularly on edges and curves where I suspect the gelcoat is thinner. I'm curious what the experts would recommend. I presume you would recommend a version if the article above. Is there an option to fill pinholes and compound over?

Repainting is an option, but a very expensive one. On a previous 36 ft. ketch rigged boat I was quoted $32k for a professional deck repainting (including all hardware removal and rebedding). Repainting a deck must be the most expensive project on a boat - far more than a new engine - and the deck takes more wear and is the one place every owner sees unlike the topsides.

I look forward to learning from the experience of others.

Thanks.

Posted by: Holt B | March 20, 2014 8:36 AM    Report this comment


Add your comments ...

New to Practical Sailor? Register for Free!

Already Registered? Log in

Forgot your password? Click Here.