So which varnish is the best to use?
When I first started out making items with polymer clay I used a bit of an unorthadox method, and finished my pieces with clear nail polish - it makes a really hard finish and it your pieces are small like mine, it keeps little bits from chipping of since it acts like a really strong glue.
This varnish is fantastic - its best feature is it's a really thin liquid, but still makes an extremely hard shiny shell over your piece that protects is really well. Two layers of this will make your items really last. I haven't noticed any yellowing or peeling, either.
One drawback is that it is a slightly whitish opaque colour, and if some pools in a little crack, you can be left with a whitish mark where it dries. To avoid this, use the paintbrush in a dabbing motion and only apply a thin layer at a time.
Since it worked so well, but is a bit pricey, I tried out another polyeurethane varnish from ebay a while ago. Unfortunately it just didn't cut it. It was by Craftmill and quite a lot cheaper, so I thought I'd give it a go. It ended up, when in contact with glue, turning crystaline and crumbling off. It also had a tendency to yellow a bit. I stopped using it immediately after a bunch of my teapot earrings broke in transit to a shop where I sell some of my items :( Into the bin it went!
Another popular polymer clay varnish is by Sculpey, who make Sculpey polymer clay as well as Premo, which is my favourite. I haven't tried it myself, but it will be my next to experiement with. I've heard it can be a bit goopy and peely, and that it has the yellowing problem, but I can't judge as I have yet to try it. I also imagine that it will be slightly cheaper than Fimo, as most things are!
There are lots of alternatives for polymer clay finishes, from floor polishes to wood varnishes, to paint varnishes that are common in craft stores, and there seem to be varying results in a lot of them.
Personally I'm going to stick to the purpose made ones, as I want to play it safe with jewellery that I'm selling. It's hard to know how a varnish will hold up a year from working with it, and since I know Fimo works well, that is that one I will probably always stick with. It's hardness always impresses me, and it may be a bit pricey, but the confidence I have in it is worth that price!