One tip I can give that works with regular miliput (not sure if superfine works quite the same), is to have a glass of water handy when you're using it. Once you've got the miliput mixed and on the area you want to fill dip whatever tools you're using to sculpt it (I tend to use an old exacto blade, a few dentist tools and my fingers) in the water from time to time. Miliput gets really sticky as you work it and its makes it way easier to smooth it down if you moisten your tools.
Depends on the size of the crack/holes. for small stuff I use spot glazing putty... Its like fine bondo in a tube.
Jedi Dade
Then I'd go to the auto parts store and grab some "spot glazing butty" in the paint area... its perfect for that kind of thing. I use it to seam all my models and fill small gaps. The last time I bought it - it was around 5-6 dollars per tube.
It dries pretty fast but make sure you let it dry before sanding... or you'll be doing it again
this is the stuff you're looking for:
http://bondo.com/bondo-glazing-spot-putty-907.html#stw3LQ2L6GHsZk6o.97
Jedi Dade