Hello BD63,
First a disclaimer. I can't answer this question. But I'm sure you'll get a lot of input, and I too am interested in hearing the responses. I believe you need to clarify, however, if you are referring to actual water slide decals or the new reproduction type that I believe, are vinyl or something. I think someone recommended spar varnish because it does have a higher solids content and fewer solvents than other clear finishes. That's why it does not completely "harden" and why it is the preferred finish for bamboo fly rods; it stays somewhat flexible. In a relative sense, compared with other clear finishes. And yes, it will tend to yellow over time. I can only recommend, for what it's worth, that you try out some different options if you have any left over decal material that came with your replacements. I might tend to airbrush some clear poly urethane over decals applied to a painted test surface and see if the solvents lift the vinyl edges at all. Good luck and please keep us posted.