This may not be new to come but it was to me so I thought I would throw this one out here.
I was browsing some site when happened upon a tip that caught my eye.
If you have problesm with paint bleeding under the masking on your project. The solution is to paint a small amount of clear gloss over the edge of the mask and model.
The clear will then prevent the paint from "bleeding" under the edge of the mask.
I haven't tried this yet but, it is filed away for a vert near future use.
Hope this helps someone else out there besides me.

Paul