I was at home depot and I asked them about the best way to waterproof wood and I described the signs I was making and I was told I needed “polyurethane” or ‘spar varnish’. Then I learned there were many types. Oil based, water based, interior, exterior.. Since I wanted it for outdoor use I had to get the exterior one.
I ended up with this:
As the directions state, you have to coat it several times. I applied three coats, lightly sanding if needed. In between coats I wrapped the brush in plastic wrap or a ziplock bag, although I suppose I could have obtained spirits or whatever would clean the brushes.
Note: if anything gets in between coats, then it is stuck there! I also made the mistake of leaving my signs out to dry when it was lightly raining and little pock marks ended up on the signs from the droplets of rain.
I did my best to go in one direction and the final results were pretty good.