I do use spray satin poly on my hunting bows to cut the shine of the Tru-Oil. I have use it as a more complete coat over Tru-oil also with no problems. Tru-oil in multiple coats will protect your bow just fine. No matter what you use you will never make a wood bow moisture proof(resistant, yes) and as a matter of fact you don't want to completely seal a wood bow. It need to breath or it will dry rot.
I have sat through a few hours of moderate rain with no ill effects to my Tru-Oil finished bows. I have bows that change with the R/H no matter what finish I've used...Tru-Oil, spray poly, poly wipe on, shellac and Massey finish or a combination of some. Pat