I wouldn't. Modern way is ribs and spars and all reinforcement blocking is bonded with West or T88 epoxy, then the complete ashy is garnished with same epoxy. Then its is bonded and sealed. If you spray PU on the ribs, you interfere with bonding between parts.
Some people still use varnish other than epoxy on small wood wings, but as was stated, you don't want to risk poor glue joints on the rest of the assembly. Some people mask the areas that will need glue, some sand, etc. Sounds like a lot of work to me, but each person, situation, is unique. Personally, if I get to building my wood airplane, I will probably use epoxy for all of it. I've been told West Systems special coatings hardener is good for that, but do your own research. My memory isn't especially awesome.