I guess this stuff must have been a scratch resistant coating or film cleaning agent which worked in the short term, but has damaged the film over decades of storage. Is this the case? Does anyone know what the chemical composition of Scotchguard was, how widely it was used or anything else about it?

New 35mm Intermission films http://www.drive-infilm.com

It can be used both on duplicate negatives and prints. It is a polymeric coating that is applied to the film and then cured (hardened) with intense ultraviolet light. It is impervious to most solvents and water, and cannot be removed.

You needed special splicing tape; regular tape wound not stick to it. In fact, very little would stick to it, including dirt. At the time (about 15 years ago) we dunked a short length from the tail leader into old pizza sauce, soda, etc. It was weird how nothing would stick. I remember thinking how someone will be digging this stuff 2000 years form now, because it will never go away!

You could see a texture to it, sort of like brush strokes, when the light reflects off of the film. Didn't see anything on screen, though.

Scotchguard and Film Guard were made for different purposes; Scotchguard kept the film from getting dirty, Film Guard cleans the film if it gets dirty.

In the days before Polyester prints some distributors payed extra for the Photoguard treatment with acetate prints they thought would get extended runs in theaters.

Filmack Studios was a big proponent of Photoguarding their snipes and datestrips.

Dacar Products Company makes similar UV-cured film coatings called Imagegard:
http://www.dacarprod.com/www/ndatesh

I must disagree here.

Filmguard is made to improve the presentation of any theatre, primarily by keeping prints clean from the get go. You can use it effectively to clean dirty prints, yes, but it was made to stop the cause, not treat the symptom.

Scotchguard (and its evil twin Imageguard) to my understanding exist to "rejuevenate" prints. It is applied only to the base side only (really helpful for preventing emulsion scratches). It does run through the projector louder and rougher. Many of the treated prints often shed like no tomorrow, actually making the onscreen presentation much dirtier and far worse. In its defense, it is near impossible to put a scratch on the base side of a treated print. And John is definitely right... it's impossible to remove!

We have run some trailers every day for years and the quality is still evident.

I have two prints of the Imax film "To Fly" which are photoguarded and date back to 1983.
Each has at least 2500 passes on them ,the colour is a little faded but not a scratch on them.