I undercoated them with white acrylic (though I have used spray acrylic to undercoat recently).
Then after painting them I used a spray Matt varnish from an art shop. This seems to have worked fine though on my D&D minis (which are made from a more rubbery plastic) it hasn't dried and they're still sticky

I'm not sure if they're gonna chip as I haven't played with them much and they are stored in a segregated way.
I drank while I was doing a few of them, but it started to make my eyesight too blurry to work so I had to stop.