If existing paint is sound, I wouldn't bother, but I'd sure still prime it! Vinegar wash any bare tin to help etching/adherence prior to priming (now that we can't buy good 2-part xinc chromate primer).
I also would use West Systems G-Flex 650 epoxy ($20-$22, goes FAR!) over Gluvit, as the G-Flex product is flexible. But ... you're only putting where you have a penetration, right?
Meaning, I would NOT use either of those products on a large panel, as WHY would you ever need to? Now with that said, on the running bottom of my hull, or an immersed panel, the rivet heads are sealed in the G-F epoxy, but then also covered by Steel Flex epoxy.
And remember, ANY epoxy product needs to be top-coated or painted to prevent UV damage.