I use a marine fairing compound like this.

I have a painted bottom, and whenever it gets scratched deep enough for me to get concerned about it I mix up a bit, putty it on, and can hand sand and touch up the paint within minutes.  It's very quick and easy.  

System 3 also makes a 2-part compound called QuickFair which is supposed to have the qualities you want.

You can make a fairing compound with microballoons, bit when it's a stiff mixture (lightest & easiest to sand) it get somewhat difficult to spread, especially to spread quickly. It curls back under the spreader. QuickFair is supposed to stay easy to spread. The only complaints I've heard about it are that it can be difficult to stir up the fillersif they've settled. But they're only in one part so there's no time pressure and once they're stirred up again they stay that way for quite a while.

You can google to find distributors. 

Laszlo