Your best bet is to always stay with the same manufacturers base and clear.There are some compatible to intermix,but it's not recomended unless you know someone whose done it and has had a sufficient timeframe to verify it.The incompatibility will show from immediately to a year away depending on circumstances.PPG has done me right for the last 8 or so years,but I'm a touch off the conventional approach.It's nothing for me to mix clears to get what I want,but I advise against it because you can screw up $100 worth of clear easily.Remember this about clears (or single stage color),the faster it dries the more brittle they are and if you choose a force dry clear you better have a way to cook it or you won't like the results.We have 3 PPG clears in the shop now-2042(fast),2021 and 2002(slowest).I prefer to work with the 2021,but have done the initial work in 2042 and recleared later on with 2021.Here's a prob on my bike.I bought it off one of the 'painters' at work and he recleared the tank to cover the decal edge.The prob is he pounded about 8 coats of 2042 on it in 2 days (baking each 2 coats) to eliminate the edge over a year ago.I buffed it about 6-8 mos ago to remove a dirt nib and overspray from sitting around the shop.Within a month the finish continued to shrink up and in 2 mos the dirt nib came back and is still there.No biggie cause I'm stripping it when I get the time,but it's an example of doing what you shouldn't.I actually won't be surprised if I don't have the stuff want to separate or be soft towards the botton of his work.I'm by no means a chemist,and if you're not don't screw with this stuff,it's way too expensive to experiment with.