Sam - The forum has a hair cross-wise today.
Gluvit is a form of epoxy and it can be thinned a little. I don't what, if anything, can be added to thicken. UV eats it up and paint works.
Here is a blurb about painting 5200 from the 3M FAQs;
Although 3M does not recommend the application of a finishes over our sealants, some boat owners and boat manufacturer's may choose to apply one. Many finishes contain solvents or plasticizers that can harm the cured sealant by causing it to swell or soften. This may also inhibit the drying or curing of the finish.
If you do chose to apply a finish over our sealants or adhesive/sealants, we strongly recommend testing your particular finish on a small sample area before application. Never apply a finish to uncured or freshly applied sealant less than 30 days old. The finish may prevent the sealant from curing completely.
Another potential problem is cracking of the finish. The cured sealants and adhesive/sealants are flexible. If movement of the substrates causes the cures sealant or adhesive/sealant to change it's shape, a more rigid paint film or other finish that has been applied on top will not change shape with the sealant and will likely crack and eventually flake off.