Sapele with spar varnish sounds good. The thing will have a bit of a roof and be on the north side, so hopefully not too much sun and won't need once/year. Any thoughts about slipperiness when wet ?
Rabbit hole warning: you're talking to a guy whose invested time in wooden boats is 100x their invested time and interest in woodburning appliances.
I have one boat coated in 16 coats (sprayed) of Epiphanes, and I have not had to do another coat since about 2008, so 15 years. But it's kept indoors, only put in the water for 4 hours on race day, then pulled back out. In the shade, you should also get several years between coats, but that will depend on whether water can get into the wood behind the varnish and cause delamination around joints or screws. The broadside of varnish to wood itself will never fail, it's always when the wood gets compressed or wet around fittings, that you see it starting to fail.
Most of them are made to be underfoot, and although slick when new, will wear in to something less slick with a little time. However, many choose to put a grit into the top coat or two, for optimal traction. They make special grits for this, designed to float better in the finish than regular sand, you just broadcast it onto a wet coat, either freehand or using a pizza shop style garlic shaker.
"Spar varnish" is the term for varnishes made for marine application, "spar" being the generic term for any stick (mast, boom, bow sprit) on a boat. One key feature is the inclusion of phenolic or other UV inhibitors, to protect wood from sun, but there are also other modifications that make this product more suitable than regular varnish/urethane for outdoor/wet use.
There are two basic classes, 1-part and 2-part, and I'll outline some of their pros and cons:
- Traditional 1-part spar varnish, eg. Epiphanes
- No clouding or breakdown in full sun, ever. Just gets more beautiful with age.
- Easy application, just brush it on, like oil-based paint.
- Available in oil or water base, but of course oil is better in every way except recoat timing and odor.
- Softer, especially when "young". Gets harder with age, like alkyd paint, but never as hard as 2-part.
- Very easy to repair, just sand paper to feather out around chips / delaminations, then burnish the whole area with 240 grit or synthetic steel wool, and recoat.
- Usually 6 - 12 coats on new wood, will build a durable thick shell, as you've seen on old boats or country club picnic tables.
- Boats left in the sun, or crashed into each other every weekend on the racecourse, usually get top-coated every year. But I have one boat that's never crashed and kept in a garage when not being sailed, which still looks new after 15 years with no fresh coats. I suspect you will get several years between coats, unless your joinery/fastener design allows moisture to get in behind the wood.
- Modern 2-part urethane or epoxy-based
- More difficult appliation, must be mixed and applied before it starts to set up
- Very durable to abrasion and dings, very high initial hardness, no need to wait for it to mature.
- Very difficult to repair, if you're going for cosmetics (think pretty wooden boats).
- Epoxy-based systems tend to cloud and even break down in full sun, not as UV-stable.
- Never needs a recoat, but a full strip and redo might be required after a decade in full sun.
If Sapele is what suits you, I'd use 1-part traditional oil-based Epiphanes, and coat
both sides of your project. That will completely encapsulate it, and prevent any moisture from getting in from the bottom side. In shade, as a shower wall or floor, I'd expect to get several years between recoats. Knowing grip is usually only applied in the top coat or two, I'd just brush on a dozen coats with no grip and try using it. If you find it's too slick, then add grip with a single topside recoat later on.
I've only used grip on painted floors, never varnish. But the tech support at Jamestown Distributors is phenomenal, or at least they were 15 years ago, and I suspect they'll point you at the right product. In fact, you may want to just call or email them about the shower project, and see what they recommend. I had an open account there for a few years, but it's been a long time since.
Oh, one last thing... there was a variant of the Epiphanes varnish where you could apply multiple coats without sanding, if you hit a specific recoat timing. This was very handy for quicky building up a half dozen coats on new work, only sanding before the final coat or two to get rid of any dust nibs that built up. I did it on one of my boats, which was like sanding out an entire house by hand. I remember doing a dozen coats with the no-sand crap, then the final 3 coats with the traditional formula I linked above, came out great.
These two photos were taken 2008 and 2022, respectively.
boshart.com
