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I'm building a live edge bench in memory of a loved one, and would like it to last as long as possible while weathering the elements year round. I live in Pittsburgh, PA, which means we get occasional sunny summer days in excess of 90 degrees, but of more concern are occasional sub-zero (C) temperatures and lots of snow and ice in the winter. I'm also concerned about bugs.

The bench will be made from several bookmatched slabs of live-edge Cherry at approximately 1.5" thick. That includes the bench, seatback, and side/legs. Hidden lateral supports will be made from pressure treated 2x4 lumber.

I planned to do nothing to the pressure treated 2x4s. I planned to cut and sand the live edge slabs, then have the dedication laser-engraved. Then apply 7-10 coats of UV-resistant outdoor poly-urethane. Then anchor together using outdoor deck screws with a dab of 100% silicone over the screw holes (they will be hidden).

What other measures should be taken to keep this piece protected from the elements? Is there any sort of chemical that should be applied and allowed to seep into the wood? Keep in mind that any treatment to the visible decorative areas must not negatively impact the appearance of the wood.

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  • If you have the space, consider moving the bench indoors during the winter months or maybe a waterproof tarp to avoid long periods of exposure to accumulated melting snow.
    – Ashlar ♦
    Commented Jun 25, 2018 at 0:20
  • @Ashlar Thanks for the suggestion. I was hoping to be able to do that, but I want to plan for the worst. So during construction I figure it's best to plan for it to be outside continually.
    – Nicholas
    Commented Jun 25, 2018 at 13:09

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Cherry is not a durable species* as I presume you know, so a protective finish becomes a must to ensure a long life if using it outdoors.

Since you obviously don't have a problem with a fully varnished appearance that's good as this is the finishing option that will give you the best outdoor durability in an unpigmented finish.

But you should use a better varnish than I think you are currently planning on using. Varnishes intended for marine application are significantly better than spar varnishes sold to normal consumers, and other exterior varnishes sold to homeowners for use on the exterior of doors etc. can fare even worse under direct sun and with weather exposure.

Here's Bob Flexner on the subject on Popular Woodworking, The Best Kind of Clear Finish to use Outdoors.

What other measures should be taken to keep this piece protected from the elements?

I think you have it covered in essence. Just bear in mind is that the varnish, like every finish, needs maintenance as mentioned in the link above. For a quality marine varnish this means sanding down and applying a fresh coat periodically. Hard to say how frequently this will be needed for Pittsburgh, maybe every 4-5 years, but don't trust those numbers — you have to be directed by how the finish actually endures and not try to follow a timescale decided on in the abstract.

Is there any sort of chemical that should be applied and allowed to seep into the wood? Keep in mind that any treatment to the visible decorative areas must not negatively impact the appearance of the wood.

There are both commercial and DIY wood preservatives that might be used but I don't think you should worry about this. When varnishing the protective coat has to have failed, allowing water direct contact with the wood, for this to matter. And at that point you're already well past the time the piece should have had some upkeep.

If water does get to the wood you're looking at a complete stripping and revarnish, so it's vital to work towards this not being necessary by keeping up with maintenance.

Fasteners
If possible use stainless screws. Coated (and now multi-coated) screws are supposed to be highly resistant to rusting but once their finish is breached, as can happen with a single slip of the screwdriver across the head of the screw during installation, it can expose steel to the elements and rusting can begin.

*Low rot-resistance.

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  • Thank you for this well thought out answer. I already have Stainless screws. For a poly I was going to use Target Coating's EM9300 WB Polycarbonate Urethane with their CL100 Crosslinker. It sounds like you don't think this is a good idea. Are you able to recommend one that you think would be better if this isn't sufficient?
    – Nicholas
    Commented Jun 25, 2018 at 13:14
  • When you say "this means sanding down and applying a fresh coat periodically" do you mean sanding the entire finish away, all the way down to the wood, and then refinishing? Or do you mean sanding off a coat or two of the poly, applying a new coat, and then just rubbing that out?
    – Nicholas
    Commented Jun 25, 2018 at 13:15
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    No sorry, can't comment on the Target Coating product as I know nothing about how it compares to a marine-grade oil-based varnish in terms of durability, UV resistance and suitability for use on wood. I'm not sure anyone but Target will know this, unless they've done a direct side-by-side comparison. It's worth Googling for this, you might get lucky and there is one out there.
    – Graphus
    Commented Jun 25, 2018 at 13:39
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    Re. sanding down, no I don't mean sanding all of it off. As the link to the Flexner piece mentions this is primarily sanding the surface sufficient to remove any weathering defects. This sanding should also fairly uniformly scratch the entire surface, including undersides or hidden edges, although it's not vital that every spot receives the same attention (impossible in the real world).
    – Graphus
    Commented Jun 25, 2018 at 13:42
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    I don't think the latter would work in practice. I suppose it's theoretically possible to do (once or twice, before the finish gets too thin) but the slight degradation of the varnish surface is not the only defect that can occur, so when you refinish the fresh varnish has the opportunity to plug pinholes, minute cracks etc. that might have opened up.
    – Graphus
    Commented Jun 25, 2018 at 21:45

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