If you asked for the clearcoat, the s3 "clearcoat" is a very thin epoxy.

what I think you mean it the WR-LPU Topcoat, "water based varnish"

from their website (http://www.systemthree.com/index.html)
Clear Coat
Clear Coat epoxy is a very low viscosity (thin) almost colorless epoxy system that has a long pot life and cures without amine blush. Unlike some of the so called penetrating epoxies, Clear Coat contains no solvent. Furthermore, it is a very strong system when cured whereas the penetrating epoxies have little, in any, strength or resistance to moisture. Clear Coat epoxy wets out fiberglass cloth almost instantly and is sometimes used to build furniture grade strip planked hulls. Clear Coat epoxy is often used as the base coat in fiberglass gel coat blister repair particularly when the gel coat has been removed exposing damaged and loose glass fibers. It is also used as a base for varnish.

WR-LPU Topcoat
WR-LPU is a time tested water reduceable polyurethane topcoat. It rivals solvent-based paints in durability and weatherability. It contains ultra-violet light absorbers for maximum life in the sun. One gallon will cover between 350 and 400 square feet when brushed or rolled per coat. We recommend at least two coats. Allow for some overspray when spraying. System Three coatings are designed to work together as a complete system. Use System Three water reduceable coatings over epoxy and polyester resin surfaces that are not constantly immersed in water. Constant immersion means boats that are kept in the water all year. Boats that are dry-sailed or left in the water for a couple of weeks are not considered to be constantly immersed. We have achieved good results applying WR-155 primer and WR-LPU topcoat over bare wood. However, the type of wood, moisture content, and exposure to the elements will affect the long term results.