Although the link says it is compatible over basecoat and CURED laquer and enamel finishes, I'm thinking that they mean catalysed acrylic enamel, not the mineral spirits based rattle can enamel.

I would first paint a piece of PVC pipe with the same paint you use on the frame. Then use that catalysed clear on the PVC first. If it doesn't cause any problems, use it on the frame.

That way if something DOES go wrong, it'll be on a cheap piece of plastic, NOT on your frame.