Epoxy with Exterior Varnish
An epoxy sealer with exterior varnish topcoats is the most durable, but also the most labor-intensive finish you can apply to outdoor furniture. This is the finish favored by boat builders so you know it's going to last a long time. Epoxy and exterior varnish enjoy a symbiotic relationship: The epoxy forms an impenetrable moisture barrier that prevents seasonal swelling and shrinking of the wood. This dimensional stability in turn gives longer life to the exterior varnish because it no longer has to stretch and shrink with the wood The exterior varnish returns the favor by providing UV protection, without which the epoxy would rapidly deteriorate.
Apply three thin coats of epoxy. The best way to get thin, even coats is to use a foam roller cut in half. It works kind of like a squeegee. Epoxy cure times vary depending on their formulation and the ambient temperature. Be sure to use an epoxy with a long enough open time (approximately 30 minutes), so it doesn’t set up before you’re done putting it on. For large projects, mix the epoxy in small batches so you can finish an area before the epoxy sets. Also, for optimal flow out and penetration into the wood fibers, make sure the epoxy you use doesn’t contain any thickeners. Always read and follow the instructions that come with your epoxy. If possible, apply the epoxy undercoat prior to assembling the parts. You can recoat without sanding while the previous coat is still soft but not sticky. If the epoxy seems uneven or bumpy, allow it to harden. Then, sand it smooth and apply the next coat.
Before applying the varnish topcoats, I use a card scraper or sanding block with100 - 120 grit sandpaper to level the cured epoxy (see photo below left). The sanded surface also provides some tooth for the spar varnish to adhere to.
Rinse the sanded epoxy with clean water and dry with paper towels. The rinse water should not bead on the surface. Beading indicates that contaminants from the epoxy curing process are still on the surface and could interfere with the varnish bond. To remove the contaminants, wipe down with mineral spirits and dry with paper towels or a rag. Follow this with eight coats of exterior varnish, sanding lightly between coats.
Source information may have changed since the original publication date.
Epoxyheads, epoxyheads.com, 866-376-9948, epoxy resin #7710
$33.00/qt; epoxy hardener #7720 $17/ 1/2 pt.; metered pump set #7801
Penofin, penofin.com, 800-penofin,
Original Blue Label Oil Wood Finish, $15/qt.
Depot, homedepot.com, 48 oz. Chapin multi-purpose hand sprayer
model 1002 $8.
Home center/hardware stores, exterior oil varnish or
This story originally appeared in American Woodworker April/May 2007, issue #128.
April/May 2007, issue #128
Purchase this back issue.