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