AMMO by MIG Accessories - Masking for Modelling
Item: MASKING SHEETS 1mm GRID. x5 SHEETS. 290x145mm (Adhesive)

From the Manufacturer:

Masking sheet of 1 millimeter for airbrush custom paint design. This material allows the modeler to both draw and scribe on the surface, making geometrical figures simple to mask. This masking is easily placed correctly on the model by using surface details as reference points. The 290 x145mm allow you to make any design and exact cut with a French curve, template, or custom pattern. Due to its light transparency, you can also place it on a drawing or surface and trace and copy a design. Millimetric Masking allows you to use all types of paint; the high quality material prevents the mask from leaking and will not leave residue or a trace of adhesive on delicate surfaces and base colors. This masking sheet is easy to work with using scissors or a sharp blade, even on a clean fresh and sharp model where necessary.