Are you painting with enamels or acrylics??
Micro Mask will peel away from enamels quite easily, but with acrylics it has a tendancy to stick fast, and stay stuck!!!
I would not apply MM to metal without some ort of primer first, does the same as with acrylics.
This is what has happened to me in the past, and to aid lifting the mask up, try sealing a small piece of paper or tape in it, makes life alot easier when you try and remove the mask.