You'll not get Humbrols through an airbrush without some sort of thinner. It's like treacle in the tins but once you add the thinners then a little goes a long way. It's really important to make sure the paint is thoroughly stirred and mixed in the tin before you mix it with the thinners because a lot of the pigment can hide in the bottom of the corners. I usually spent a good five minutes stirring each pot and really scraping the bottom.
I used all Matt colours from the adpated Terminal Fettler list and they're all Matt (except silver of course). I didn't feel the need to add another coat to protect it with a clear coat or anything like that. I thought it was pretty hard wearing as it was. Recommend about a week for the silver to properly dry, maybe more; depends on climate and conditions etc.