When to sand new gel coat?

[Flightsport]

Rookie boat owner here. Just applied a quick and dirty layer of gel coat to the bare glass-repaired rudder of my Columbia 26. It seems to have hardened over-night. Whats a good rule of thumb for when I can wet sand?

note: gel coat for sealing only, not necessarily for looks.

[EdHouston]

Rookie boat owner,

Gel coat is not the best barrier, you would be better with a west system Barrier coat, this can be applied over your gel coat to eliminate water penetration.

A proprietary blend designed to improve cured epoxy's moisture-exclusion effectiveness. 422 is used as a barrier coating additive to help prevent gelcoat blistering. 422 also increases the epoxy's abrasion resistance. Cures to a light gray color. Add to mixed resin/hardener at the rate of 15 to 20% by weight–3 tablespoons per 8 fl. oz. epoxy (approximately 32 oz. per B group).
West System epoxy is a high-quality, two-part epoxy that is easily modified for a wide range of coating and adhesive applications. It is used for construction and repairs requiring superior moisture resistance and high-strength. It bonds to fiberglass, wood, metal, fabrics, and other composite materials, and is especially suited for marine applications.


All epoxies are not created equal. There are big differences in formulations, quality of raw materials, the suitability for building and repairing boats and the toughness to stand up to marine conditions. It's easy to market an off-the-shelf industrial epoxy product as a marine epoxy, or formulate an epoxy with one or two favorable characteristics, while sacrificing other important characteristics.
Good science is at the heart of WEST SYSTEM epoxy. Comprehensive testing is the basis of that science and is essential for the development of improved epoxy formulations and better construction and repair methods. Twice as many people at Gougeon Brothers are involved with technical service and testing than with marketing. When professionals need answers to tough questions, they come to us. They know our answers are based on good science and hands-on experience, not on marketing.

105 Resin is the base material of the WEST SYSTEM family of products, on which all of the WEST SYSTEM compounds are built. The resin is a clear, pale yellow, low-viscosity liquid epoxy resin. Formulated for use with WEST SYSTEM hardeners, it can be cured in a wide temperature range to form a high-strength solid with excellent moisture resistance. It is designed specifically to wet out and bond with wood fiber, fiberglass, reinforcing fabrics and a variety of metals.
An excellent adhesive, 105 will bridge gaps and fill voids when modified with WEST SYSTEM fillers and can be sanded and shaped when cured. With roller applications, it has excellent thin-film characteristics to flow out and self-level without "fisheyeing." The resin cures clear so that you can achieve a natural finish when coating with varnish. It has a relatively high flash point and no strong solvent odor, making it safer to work with than polyesters. Resin viscosity is approximately 1000 cps at 72°F (22°C).For each container size of resin, there is a corresponding sized container of hardener and a corresponding mini pump size. When purchasing resin, hardener and mini pumps, be sure all containers are labeled with the same "group size" letter (A, B or C).

[sailingdog]

I'd second using Epoxy rather than gel-coat for any underwater fiberglass applications. Epoxy is less likely to allow water through osmotically.

[jjns]

As you have already applied the gel coat, overnight should be sufficient for sanding. I would second the epoxy over top, but only after you have fared your Q & D coat. Preparation is everything and epoxy is not fun to sand.