I've done a couple playfields with Varathane (water based) about 10-15 years ago (for a while it was 'all the rage' - I think butterflygirl24 uses it as well) - there was a learning curve to it. Cliffy (passionforpinball) used to have a page talking about how best to clear PF's with the varathane - his advice was the best, because he recommended you follow what the can says to do!!
A lot of people at the time were spraying a coat, waiting 24 hours between coats, and spraying another. This is great advice.... if you like cloudy, milky, playfields!! I did a few that way before seeking out a better way. The most successful was when I followed the can - between 45 min-2 hours for more coats. I think I did 4 coats on a Countdown PF - while it's not 'perfect' - it played well and has held up for at least a decade fine. It looks nice, just as good as original clear (which is lacquer on most older PF's)
If you check on rec.games.pinball there's a LOT of discussion on results and how to do it. If I ever get back into clearing PF/or needing to clear I'll probably either get more varathane (I have some from 10 years ago, doubt it's any good....) or maybe buy all the safety equipment and shoot spraymax 2k. I no longer have a garage to work in though so I'll either have to shoot outside or use the shed (it's full of crap right now though).
Some people have been successful using varathane out of a regular paint can and foam brushes to clear PF's as well. If you have a scrap playfield around it's a good thing to practice on.
Found the cliffy link: