Which fairing compound should I use for repairs below the waterline?

Fairing refers to the filling and shaping of low areas so they blend with the surrounding surfaces and appear 'fair' and level to the eye and to the touch.

Generally, an epoxy fairing compound is best for repairs below the waterline because epoxy is 100% waterproof when cured, unlike polyester and vinyl ester resins. Cured epoxy fairing compound also forms stronger bonds than polyester or vinyl ester fairing compounds.

You can make your own epoxy fairing compound by mixing fillers such as silica thickener (Cabosil) and phenolic microballoons (to make sanding the cured surface easier) into mixed epoxy resin. Or, you can use a pre-mixed, two-part epoxy fairing compound such as Interlux Watertite Epoxy Filler, TotalBoat TotalFair, and Awlgrip AwlFair.  You still have to mix the resin and hardener together, but the resin has been pre-thickened with fillers, so you don't have to worry about getting the right consistency, which saves time and hassle.

Before selecting a fairing compound or repair putty, carefully assess the product's suitability for your repair. Considerations include:

• Location of the repair (above or below the waterline)
• Purpose for the repair (cosmetic or structural)
• Coating used to finish the faired surface

Epoxy fairing compound is ideal for fairing large and small areas, above and below the waterline, on fiberglass, wood, and metal surfaces that will be finished with primer and paint. It can be used for cosmetic repairs and, due to its superior cured strength, is recommended for structural repairs.

Polyester or Vinyl Ester fairing compound is great for filling gouges, dents, and dings or for overall fairing, above the waterline, on fiberglass that will be primed and painted, or finished with gelcoat. However, some vinyl ester fairing compounds, such as 3M Premium Marine Filler, can be used above and below the waterline. Always check the product label so you are using the right product for your particular project. Polyester and vinyl ester fairing compounds are used for cosmetic repairs only.

As a final note, it is possible to use a polyester fairing compound on repairs below the waterline, but if you do, the cured material must be overcoated with an epoxy barrier coat primer.