[cellardoor]

Hey man, i'm trying the exact same thing myself.
I've sacrificed a Jim Lee Wolverine to test some Vallejo paints on. That milky consistency guideline doesn't always work. I would experiment with some different mixes of paint and water. It's a fine art getting a mix that covers well but doesn't obscure the sculpt detail.
If the paint looks like it's on too thick then clean a brush, load it with a little water and then re-brush over to spread the paint out.
The thing i'm learning is that multiple coats are absolutely vital. The first coat is not going to cover perfectly at a watery consistency. Once it's dried the second coat sticks much better and then the third and fourth coat apply perfectly.
I've even managed to repaint over the yellow of the costume with a silver wash with multiple coats. However, I did have to fight and experiment to get the right consistency.
My top bit of advice, if it looks like it's going wrong then wash the paint off!
But don't panic. Be patient and persist and eventually you'll crack it. It's not an easy thign to do and my respect for top flight customisers has gone up a lot since trying to do the most basic repaint. It takes a lot of thought and consideration and experimentation to get it perfect.