I have used Millliput for a very longtime but I swear by it, yes it is a pain to work with in that it is hard to mix, and hard to blend, but once it sets it is very easy to sand. With Milliput, you need to take your time and work with small amounts at a time, us a knife and cut a small amount from each stick and mix it well. You will need to use your hands to roll it out, kneed and mix it together very well with your fingers. the more you mix the putty together, the softer it becomes, if you do it right, the putty will set in 3 hours.
Milliput is safe to use with your hands, but you will need to wash your hands after each use, the putty will start to set on your fingers.
I have used Microtool (this was repackaged red auto-body putty)and Squadron White putty, both are ok for filling in seams, if you want to go really cheep, plumbers A+B putty from the hardware store will work to, but Milliput is better.
