Penetrating epoxy sealer (CPES)

Overview

Penetrating epoxy sealers are thin, low-viscosity resins meant to soak into wood and harden inside it, strengthening soft or rotted fibres and leaving a firm surface for paint or varnish. They come in two broad types. Smith & Co.'s Clear Penetrating Epoxy Sealer (CPES) is solvent-borne; the maker says the solvent blend carries the resin into damp wood and that wood naturally holds 8 to 15 percent water or more. WEST SYSTEM's Wood Rot Repair Kit is a two-part epoxy that the maker describes as 100 percent solids, solvent-free, and formulated to move with the wood. Both are mainly for repair and for sealing exterior joinery and boats, not for furniture.

Pros

• Consolidates punky or rotted wood so it can be sanded, filled and painted; Smith says paint or varnish bonds better to the treated surface (maker claim).
• Soaks in rather than sitting only as a film (Smith says one impregnation almost entirely seals the porosity of sound wood, a maker claim).
• Gives a stable base under paint or varnish on boats and exterior joinery.
• WEST says its repair epoxy is solvent-free, so it avoids solvent shrinkage and voids (maker claim).

Cons

• Not a finish. WEST's finish-coating guidance says epoxy needs a finish coating for sunlight protection, since UV protection of an epoxy barrier depends on the finish coating, so plan on paint or UV-filtering varnish (Spar varnish).
• Permanent: treated wood cannot be returned to bare wood, and later repairs are hard to blend.
• Solvent versions are slow to flash off; Smith says to keep rain or dew away for at least a day (maker instruction).
• Epoxy handling needs care (see Safety), and cost per area is high.

Suitability

Used on exterior sills, window frames, boat parts and other Douglas fir, Western red cedar and Southern yellow pine carpentry that has gone soft. Smith describes it for restoring and preventing weather-related damage to exterior wood. Not intended for fine furniture or interior display woodwork, where a clear film such as Shellac or an oil is preferred. For epoxy as a thick surface coat, see Tabletop epoxy.

Coats and application

Follow the maker's instructions; they differ. For rot repair, remove loose material, make sure wood is dry enough for the product chosen, then saturate. Smith says the long pot life keeps the product penetrating for longer. WEST's data at 72 F (22 C): working time 30 to 40 minutes, recoat of a thin film after 12 hours or more, and sanding after 24 hours; the minimum cure temperature is 50 F. Surfaces then take filler and a UV-resistant finish coating. See Sanding schedule.

Compatibility

Smith says its sealer is meant to be followed by paint or varnish. WEST says its finish coatings, paint or varnish, go over epoxy to decorate it and protect it from sunlight, and lists alkyd, one-part polyurethane and other systems with differing UV resistance. Test a patch with the intended topcoat. Surface oils, wax or old finish can block adhesion, so clean the wood first and follow the maker's prep (Degreasing oily woods).

Care and repair

Keep the topcoat sound. Once paint or varnish fails and sunlight reaches the epoxy, WEST's guidance indicates the epoxy itself is at risk, so recoat before the film breaks down. Local failures can be sanded and recoated; deep damage needs a new repair.

Safety and environment

Uncured epoxy and its hardeners can irritate skin and eyes and can sensitize some people; WEST publishes epoxy health and safety guidelines covering overexposure, and both makers publish SDS documents. Wear gloves and eye protection, ventilate, and follow the SDS. Solvent-borne sealers are flammable and need ventilation. Do not use these on surfaces that touch food. Hardened epoxy waste should be handled as the maker directs.

Related

• Tabletop epoxy: thick-pour epoxy.
• Spar varnish: UV-resistant topcoat.
• Degreasing oily woods: oil removal before sealing.
• Outdoor finishing: exterior systems.
• Sanding schedule: preparation.