A quick question if anyone can answer. Can you paint over varnish?
The reason I ask is that I want to stiffen cardboard. I've spray painted it a few times and even given it a couple of layers of hairspray after that but it's not quite as sturdy as I'd like.
You will be able to paint over it just fine. Give it a light sand with a fine grade paper first to key it though. You will find it may want to soften as you paint it if your putting a lot of product on in one application or one with a high solvent content. If that happens just do light coats.