Charles, Frisk Film, often called Frisket, is a semi-translucent, almost clear, polyester film base (like photographic sheet film, without an emulsion) that was designed for the graphics industry. It is used in that field for masking areas of drawings or film/film negatives. It's got a low-tack, self adhesive back, so that it can easily be peeled off, re-positioned etc. It's an excellent material for masking some areas or components in modelling.
Until I saw Lucky's post, I'd forgotten I have a few sheets of it somewhere!
The slight problem is, it was relatively expensive for modelling use, as paints made it unusable 'second time around', and it used to come in packs of 50 or 100 sheets, in various sizes from A4 up. However, by todays modelling standards, considering the prices of most other modelling materials/accessories, it is probably not as expensive as it would seem. It is very useful material, that can be cut into small shapes, for example, for masking canopies etc. I'll have to try and find the few sheets I have left, from my time in the 'trade'.