Water Putty vs. Milliput

I've been using Durhams for years for ground work only. Like you said, its cheap and does a great job when mixed with Elmers glue to make sure it stays put on the wood base.  I don't think it will be as good as the milliput when it comes to adhering to plastic. The white milliput will give you a smoother finish fthan the other colors  when blending it to the plastic.

Instead of Milliput you would be better off with Magic Sculpt or Aves.

Folks that do Garage kits say that are smoother and easier to work with.

I use Magic Sculpt but when it runs out plan to try Aves since it is supposed to feather on the edges better, both can be smoothed out with water or Aves has a Safety Solvent that can be used instead.