I use the Liquitex high gloss varnish on wood and clay, but I suspect it's suitable for other types of surfaces as well. It is transparent and doesn't have a toxic smell (like some other do)- it actually barely smells at all. I usually use it with a painting brush (you need to wash the brush before the gloss dries though) and find that two coats are enough (although the bottle says you should use three).
I've always loved gloss varnishes because they make the colours of my crafts pop. If you look at the photo above, the figurine on the left is varnished and the other one isn't. Their faces have been painted using the same acrylic colours (not the hair though) - I think the difference is noticeable.
This one dries in about an hour (or even quicker, depending on how thick your layer is), but I always let it dry for a bit longer just to be sure that I don't get any fingerprints on my stuff.
You can find this one in most craft stores around the UK. I paid around £7 for a small bottle a few years ago.
PS: This post is not sponsored.