I usually use the Tamiya masking paper for the transition. Behind that I use cheap masking tape that I find in DIY stores. Sometimes, for large areas, I also use "food plastic film" (the one you cover the dishes with) but there, take care ! I once used special plastic film that is compatible with the microwave oven : the problem is that this film has a number of microscopic holes in it : you don't see them but the paint will go through. I got a very strange effect : very small silver points on my black paint job !!!
