I was at my local art supply store the other day and ran across the product below. Liquid mask is a life saver for anyone painting their armor, helmet, etc. using a layered painting technique. Going through the different threads, I've seen a good number of people using Winston & Newton Art Masking Fluid. I used this product with my last bucket and it turned out great. The problem arises from getting the level of detail I wanted using a paint brush or nib, and also the fact that even the opaque version of this product was pretty hard to make out once it had dried (especially on lighter colored layers). So I wouldn't be able to discern if I'd goofed until after the paint was applied and I was peeling the masking off. So I started thinking that a technical pen might work for my purposes. While out looking, I stumbled upon Masquepen. At this particular art store, it was located in with the fabric paints. A bottle runs about $13, but I've almost done my entire helmet with the one bottle.