Waterproofing with Epoxy

Waterproofing outdoor furniture may be a losing game. But if you're using epoxy to protect exposed wood, choose the right product. May 16, 2014

(WOODWEB Member):
I am starting to make wood furniture for outdoor use. I want to use epoxy to waterproof the wood before applying poly for UV protection. I am looking for references to facilitate my learning. Also, can epoxy can be applied over spar urethane?

Forum Responses
(Finishing Forum)
From contributor B:
The epoxy makes a great sealer, but it needs to be protected from UV. This is why you generally need to apply a topcoat over it such as a marine varnish, or urethane, etc.? For particular products/schedules I would check out some of the boat building sites and suppliers. 2K urethanes also have a lot of potential.

If this furniture is going to be used seasonally, and/or placed in the shade with minimal exposure, I would try not to go overboard. Keep it simple and use an exterior oil finish that can be reapplied, or stick with a marine varnish or exterior urethane.

Just remember no matter what you do that the UV radiation from sun exposure wreaks havoc on transparent coatings on wood. Regular maintenance will be required. Re-coating every 3-5 years, more or less, will be necessary. Pigmented stains or paints add a tremendous degree of protection.

From contributor R:
Having worked on both brightwork and custom Adirondack furniture for many years, I would suggest using CPES and a UV inhibited phenolic resin/tung oil varnish. Both are wonderful products for your applications.

Why would you want to apply epoxy over spar (urethane?) anything? You're sort of missing the point.

From the original questioner:
Thanks. My client had this piece built and it was sealed with urethane. Moisture has caused problems. I'm wondering if epoxy can be applied over the urethane so that stripping the whole piece can be avoided.

From contributor R:
If a varnish (this includes urethane resins) has been damaged by moisture, the only good way to restore it is to remove the coating down to bare wood and re-coat it. If you were to try and just cover up the damage, it isn't repair, it's just obfuscating the problem (and delaying the eventual fix).

When boats are repaired, the wood is scraped and sanded to bare wood. I always seal it with CPES and then water won't be a serious issue again.

So what one can do is not necessarily what one should do. It does, however, differentiate professionals from DIYers and amateurs.

The problem I have with polys is that they are not very good at inhibiting UV degradation and they have not proven to be very good at standing up to lots of moisture. This is changing as new 2K polymer resins are being incorporated into marine applications.

Phenolic resin and tung oil (drying oil) varnishes have been used for a very long time. They are also very good, especially for marine applications. Here's the problem with most spar varnishes (including polyurethanes). They are not true spar (marine) varnishes. I'd say maybe only 10% of spar varnishes (maybe less for polys) are even adequate for their stated/intended use, let alone preferred.

So, unless you're up for reinventing the wheel, learn from history and keep an open mind to improvements in materials and methods. New is not necessarily better. Sometimes, yes; sometimes, no.

From contributor D:
Once you put a barrier coat on an outdoor item, you are chained to it forever, maintenance-wise. Barrier coats for exterior clear finish woodwork all started with boats, and those finishes are designed to be removed entirely and then reapplied regularly.

Think about your epoxy/urethane coated pieces and what you will do when given a piece back with peeling epoxy. You will need to remove all of the topcoats of urethane, then the epoxy in any damaged areas - down to new wood, then reapply, dealing with old wood color versus new wood color, etc. Is this something you - or your customer - want to do? Annually? This is why the best outdoor furniture is built with oil or no finish, and left to weather.

From contributor I:
Look at the West System epoxy web site for help. Wood moves, thick epoxy doesn't. Small cracking will happen from this movement or UV failure. Get any small cracking, and you get water under the finish. Then you have the finish failure. If you want clear finish on wood, expect lots of maintenance. Nothing waterproofs wood - Mother Nature always bats last!

From contributor L:
I've had good luck with the West System. If you plan on putting a coating over it, make sure you use the 207 hardener.

From contributor R:
Guys, West System is a marine epoxy, but it is not a penetrating epoxy. Clear Penetrating Epoxy Sealer (CPES) is made to do this, nothing else (other than rebuild damaged sections of wood). I like West, I've used it at least 50 times, but it is good for what it was designed for. It is not a sealer. Read their lit. It makes maintenance much easier and it doesn't come off with sanding. That's where the penetrating comes in.

From contributor I:
West 105 is called a barrier coating and when applied per spec, it gives a 3-4mil thick coating.