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I made this sign by gluing numbers made of birch bark to another piece of bark:

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This is for outdoor use where it freezes hard. The numbers are about 1/8 of an inch thick, and water will collect in the recesses and freeze and thaw until the sign is destroyed.

I wanted to protect it from the elements by coating it with many layers of spar urethane, until the layers flow around the numbers until smooth, but after about 10 coats, it's not happening.

I know I could use epoxy resin, but that would give it a glass block finish that doesn't really go with this kind of rustic look.

How should I coat the sign to accomplish my goal?

  • What you've applied already should provide a good start, but TBH if you'd used a reliably waterproof glue you might have been able to get away with coating it with nothing, since bark is already weather-resistant. How long do you want realistically want this to last? – Graphus Jun 29 '18 at 12:28
  • Re. the varnish you used and already having applied 10 coats, this would seem to be a good illustration of what I referred to at the start of this recent Answer. – Graphus Jun 29 '18 at 12:30
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Making wooden products for outdoor use is about more than just the finish, it's about the design. Everything you make needs to avoid water sitting on flat surfaces and/or being sucked into small gaps and cracks by capillary action. I design conservatories / garden rooms etc. and any horizontal face which may have water land on it has a 9 degree or greater angle on to encourage water run-off, to achieve the highest durability (we aim for 5 years before needing a re-paint).

That said, something like Yacht Varnish should protect against this - the main issue is that these kinds of varnishes need to be re-applied at least once a year. Ultimately, if you've adequately sealed all of the gaps and nooks and crannies, you might well be alright. 10 coats of urethane sounds to me like it will be enough but I don't personally know what the performance of urethane is like in regards to heat, cold and UV light.

If you can, I would suggest mounting the sign with the main visible face slightly angled downwards from the vertical plane, as this will help to stop water sitting in the cracks.

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