Guess that would depend on your climate. I can never get SPF to hold much of a finish in NW PA after the winter. Usually Poly just flakes off after a season. White Oak and Larch seem to do pretty well though. As long as you glue your jointed boards with a waterproof glue like Titebond III you should be good. Assuming you’re using plain sawn boards for the larger sign alternating the growth rings each board will help keep the sign flatter. Just make sure your surface grain is going the same direction for each board to prevent tear out when planing. If you want to go the extra mile you could breadboard the both ends of the sign to keep it flat as well.
I have worked signs a bit. Now here is the thing. You really need to work out your process. Last sign was in freezing shop. Very difficult. The paint really did not cure. Oramask worked but in spots it removed my paint. Signs (paint) cannot be done in the pace i was working, but i did a great job but my work was 2 times more difficult. All my signs are carved so i paint, mask, cut, paint cuts, wait for a bit and peel. Now the next type signs i do are going to be multiple pieces glued. So i can paint ALL at the same time. Make things easier. Hope this helps feeks like i am rambling.
On my chainsaw carvings (some of which are signs) I use only Minwax Helmsman Clear Semi Gloss. It’s a marine grade spar urethane. Two coats minimum - the third coat gives you a “sheen” so I’m often selective about where I give a third coat. Then I leave it with the homeowners to put a fresh coat on every 12-18 months.
It does sometimes flake off in spots or more accurately I think it soaks in and leaves the surface wood exposed which then goes grey if not tended to. It never flakes off like I would imagine paint on a fence flakes off. The real secret to this stuff is that it contains a very effective UV inhibitor that prevents the wood from going grey. Also any colour that otherwise might fade from UV get protected if you apply the Helmsman over top.
Very early on in my chainsaw carving learning I had the opportunity to head north of here a good 10-12 hours to a place called Chetwynd. They have a massive chainsaw carving competition every June (I think it’s June) and the pieces - like ten or twelve each year - remain in the town. There are hundreds of massive chainsaw carvings up there. All over the town. Anyhow…I was invited up to be the assistant to a carver from the UK and went with one single mission - to find out what Chetwynd used to protect their carvings because I had been trying everything and nothing was working very well.
Answer - Minwax Helmsman Clear Semi Gloss. Have never used anything else since. It’s not cheap but you definitely get what you pay for.
[ Edit to add - most all of my carvings are spruce pine or fir - that’s all we get around here that grows big enough to carve the stumps left behind. A lot of sign jobs I’ve done were actually lumber, biscuit joined and then carved with the chainsaw so all signs of “lumber” disappered. If you dig through my posts you should find at least one sign I used the MPCNC and a chainsaw in tandem to create. ]
Epoxy is wonderful but costly and you must ensure the wood os as dry as you can possibly get it as far as I understand. I’m not speaking from experience here just based on what others had told me in the carving world (where we often deal with green woods).