Are there any archival quality/acid free polyester (acetate) films suitable for use with inkjet printers? Images printed on these will then be used as overlays over lithographs on acid free paper. Is this assemblage process compatible – would it be best to fix the lithograph with a protective spray? Grafix claim their acetate printable sheets are acid free – but they only produce A4 sheets for craft purposes. There seems to be mixed data on the archival properties of acetate as it is not a common method for fine art use.
Just found reply to orginal query – thank you so much for your advice and information shared
I think you might be confused about polyester and acetate – these are two different plastics (aka polyester most certainly does not mean acetate). I googled the Grafix film and it is made from cellulose acetate, which is laughably unstable. Cellulose acetate degrades easily, releasing acetic acid. The substrate itself yellows, shrinks, and cracks (see 
for an example). This process you probably have heard of as vinegar syndrome, and is a huge concern for the motion picture film industry.
https://www.imagepermanenceinstitute.org/resources/newsletter-archive/v12/vinegar-syndrome
Fun fact, the conservators who created A-D strips, which are indicator strips used to identify the degradation of acetate film, are the only conservators to ever win an Oscar!
https://www.imagepermanenceinstitute.org/resources/newsletter-archive/v12/academy-award
Polyester on the other hand, is a pretty good material for longevity. There’s plenty of inkjet-printable Mylar sheets on the market, as well as large rolls intended for larger printers (not just A4).
I would also point out that applying a fixative is generally considered a bad idea – these typically yellow and cause their own issues (the coating may contract faster than the substrate and cause physical stress). It’s also probably not necessary – if something is printed on plastic intended for such a process then it should not have issues that would require fixing.
Alex Nichols
The safest plastic sheet is PET, mylar. This material has stood the test of time and makes chemical sense. Acetate, as we can see from its name, is modified with acetic acid, which makes it less stable and less chemically safe. To create the kind of sandwich you outline, it makes sense to find a polyester sheet that works with your printer and work on a method of securing it to the litho, without using any adhesive, since all of them will entail problems. No spray will protect safely, since all are aerosols with solvent, which means that they will sputter onto the surface and are likely to have residual solvent, so please avoid that option.
Hugh Phibbs
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