spray can & liquid mask?

[wingers]

I'm planning to (attempt to) paint my first shell, but despite lots of airbrushing advice on here, I can't seem to find much in the way of advice for rattle cans.
Much as I'd like to get an airbrush, I can't really justify it before trying a few the simple way.

I've got a design in mind that I think will need liquid mask to make it work and I've read that a this can react with spray can paints, so how much should I apply or is there anything else that I can do to help?

So far I've ordered some paints and liquid mask, and also some "Nitro block" to seal it all at the end. Would it be worth a coat of Nitro block over the mask to protect it before I start painting, or should I simply go or an extra few coats of mask when I apply it?

Wingers