I've just done my track wheels in black
You don't need any undercoat or any prep really apart from a good clean. I gave mine a wipe down with some IPA just before spraying.
One thing I would suggest is to try and deflate the tyres and get cards round the edge of the wheel.
Also make sure the edge is really clean. All sorts of tyre silicones etc get down there and the Plastidip just won't stick.
Give the wheel and the can a bit of a warm before spraying and make sure you shake the can well. Its quite a thick spray so make sure it's not too cold.
I got mine from ukdips on eBay
I'll take some photos today of my finished wheels.
This is what my first one looks like...