I’m finding lots of sources saying PAX paint is good for painting onto latex (it’s apparently often used on latex masks) but it’s not good as a top coat. It’s sticky. Apparently you can make it less sticky by mixing more paint and less Pros Aid, like 10 to 1 instead of 50/50, but I still don’t get the impression it’s a top coat.
One latex mask guide suggested painting “Matte Medium” over PAX paint as a topcoat. I wonder if Matte Medium would make a good topcoat directly on to latex too, but I can’t find any info on whether Matte Medium will go onto latex as well as it apparently goes onto PAX paint (or whether it will degrade the latex somehow). Apparently you sponge a layer of Matte Medium on, and it goes on white but dries clear. Matte Medium is available from some art shops, “Liquitex” is the popular brand.
Elsewhere I’ve heard that “Gelcoat” is good stuff for applying as a top coat over latex larp gear. It’s usually used to waterproof fibreglass boats. See this topic for further discussion. While we did get feedback from one NZ supplier that it may not be appropriate, given that some people are reporting they have successfully used it with latex I believe it’s worth trying. I think I’ve heard it’s what some professional European latex weapon companies use.