Finally attached my sideart and bexel and have varnished both and have a couple of personal opinions on both:
1) Had sideart and bezel printed on adhesive vinyl. I was really looking for a heavy card stock but the printer did not have anything I thought was heavy enough in the size I needed (sideart was 36"x60"). Adhesive vinyl seemed very promising, just peel it off and stick. I did my bezel first, since that was a much smaller print than the side arts. Whoa! That adhesive vinyl is next to impossible to use. It is so sticky that the slightest touch and it is stuck hard. So when I was mounting it, as I was trying to align it, different parts of it kept getting stuck to the wrong spot. I had to peel off parts a couple of times to get it aligned, causing the sticky stuff to wear off. It was a horror show. I am admittedly not an expert on sticking that stuff on, maybe if you've done that a lot and know some tricks it would work better - but knowing it would be hard I had it all planned out, a great idea as to how to do it without it sticking all over - but no, it was way too hard to do. For the side art, I left the adhesive vinyl on it's backing and instead used 3M super 77 spray adhesive - whoa, it was like cake. What is good about it is you have 10 seconds or so of play to move things around before things set in and gets stuck. I got both side arts up in less time than my one little bezel.
2) Varnishing. I bought Krylon triple thick crystal clear varnish in a spray can - again, I admit I may not know the best way to do it - but after 3 coats the finish was very uneven. I think it would be great stuff on something small, but my large sidearts the finish came out uneven. I went back and bought similar stuff in a can and applied with a large brush - perfectly smooth, even finish. The spray stuff also leaves a coat of varnish dust all over the house too... I inhaled a ton of it. The can varnish applied with a brush, not only was much easier and came out perfect, but there was no dust and varnish cloud moving through my house.
So in my humble opinion I would go with (a) a very heavy bond paper/card stock thick using spray adhesive instead of adhesive vinyl - and - (b) don't go near the spray varnish, use a can and a brush!