nah no polyurethane needed on mine. I just made sure that everything was stuck on their good. Went over every edge with an iron to really seal them (had some loose trim covering before, but they stayed on good so far). You can also take some trim solvent and dip a q-tip in it and then go around over all the edges to seal. No trim coming loose afterwards and works great, is easy and fast (let dry overnight) !
Oh, and yes anything that you spray on will add weight, but I really doubt it would be significant if you just use a thin layer.
Just my $0.02