We used to brush lacquer type primer on all the time. Especially if we needed to get a thicker layer faster and didn't want to waste primer or if we were priming something like a trunk floor or floorboard. A friend calls that method the HVNP approach (highest volume no pressure). With that type primer we normally mixed 50/50 primer and thinner. If we were brushing it on, we'd use a mix more like 60/40 primer/thinner but it always worked fine. It was important not to lollygag because that primer dries almost immediately. What I've learned about epoxy (of any sort...filler, primer, whatever) is if you mix part a and part b the only thing that will stop the mix from curing is if you don't mix both parts together completely. If you get it thoroughly mixed it may cure faster (too much hardener) or slower (not enough) but I've never seen anything that will stop it from curing. Oh and here's another thing: if you need to paint on a layer of genuine epoxy (like JB Weld), you can do it. Get the regular JB Weld (not the Kwik Weld), mix both parts and then thin it with acetone to a brushable point. About 6 years ago I had to repair a fuel tank on a log splitter (a tank which was no longer available anywhere) and I did this--two coats. After it set for a few days I re-joined both tank halves sealing the seam with epoxy and it is still working fine today. Good luck!
