Undercoating: I use my excalibur. A rather long peace of wood. covered in double-sided sticky tape. Sick models, Spray, unstick models.
Painting: I usually 90% of the time attach whatever im painting to the top of a old
GW Paint pot with blue-
tac. For smaller items. (as in tiny...) Pinned to a paperclip, then hold the paperclip in a small vice. Gives me something large and solid to hold while still being able to manupilate the mini.