I've used the level below extreme detail for little WW1 aircraft (ultra detail?). They paint up well, but come with a layer of wax (or similar) on top that can be washed off with warm soapy water (keep in mind that whilst its warm there is greater potential to bend the model, perhaps useful. If the wax isn't removed then the paint just won't stick (if you're using oils it won't dry either). You'll know it is clear of wax when the models become see through. Give it a bit of time since in my experience if you don't wash it for long enough you get this strange bubble effect where some is washed off and other elements aren't. Once washed, I've encountered no problems using any normal technique on them, although I've never washed them prior to a base coat or two.
As I said, this is primarily applicable to the level one step below extreme detail, but hopefully this helps.