[RobD70]

I am making a couple outdoor signs for a friends dog boarding business. My first try at it. 1 sign will be the business logo and the other the word Welcome. Both are cut out of mdo plywood, have been primed and painted, both water based. The backing is mdo plywood with cedar fence pickets glued and brad nailed to it. I used 2 applications of Thompsons waterseal on the cedar. I am putting Varathane Spar Urethane on the painted parts before assembling the signs. My question is, what would happen if i put the spar urethane on that watersealed cedar? Im guessing it wouldn't adhere to it very good. Being my first time making the sign, i didn't think thinks through very well so everything worked good together. Thanks

 

[mark sr]

The properties in TWS that make it repel water also make it repel any coating over it.
You might get oil base spar poly to adhere if you aggressively scrub the TWS with mineral spirits - no guarantees.

 

[carpdad]

Thompsons water seal got me little curious since it's been a while and wondered if it changed. The company has a sheet and the names, polysiloxane and hydrophobic wax. Siloxane seems to be associated with silicone and wax is wax. As such, without going in too deep, it looks like nothing much will stick after the water seal. Trying to remove it probably will damage the paint.
Even after many years, I still would not recommend it on anything outdoors.