Krylon K01303007 Acrylic Spray Paint Crystal Clear
I prefer my actual HQ minis to be unpainted, but when converting other minis to look like HeroQuest minis (for example, for rare expansion minis that cost $60 each on eBay, or for minis for 4 new Heroes I made) I use the acrylic spray to protect the paint. I also used that spray on my HQ box, as it protects the inked part from being worn off.
Here are my four minis for my 4 new heroes: halfling, gnome, priestess, and fighter (with barbarian in the middle for comparison). They are painted dragon red and sprayed with the clear enamel spray.