clear coated film

You probably need to explain your application in further detail. But there is a company called Sensor Products, which makes Pressurex film. Essentially the male film comprises microspheres that "explode" releasing red color when pressure is applied. The female-film is a clear plastic that holds the color. Very similar to carbon-paper, but only carbonless.

Hope that helps...

Niles

Various book-covering clear adhesive films are sold, as are laminating pouches for covering maps etc. These cover whole areas.

Thermal transfer printers / hot foil technology sounds like sort of thing that you want, though these are normally pigmented. In principle, a clear hot foil film would be easier than a pigmented one, but I don't know if there is a clear one on the market. You don't say how big your application is, but in low volumes a soldering iron might well be used as a hand-guided 'writing' device.

In practice, the issue with this sort of film is how to make a clean edge between the area where you've written and the area that you haven't.

-- Anton