Listed below are the salient points of different variants:
Semi-crystalline resin. Higher the crystallinity, higher the opacity. Depending on crystallinity and processing, products made could be flexible (PSA tapes, magnetic tapes, food packaging) to semi-rigid (bottles) to rigid .
Amorphous PET. Amorphous PET is very transparent due to the rapid cooling below Tg during processing as opposed to slow cooling of crystalline PET.
This is also clear and amorphous. The backbone is built with CHDM, a much larger molecule in place of conventional ethylene glycol, thus reducing crystallinity and lowering Tm. Used in 3D printable thermoplastics as lower Tm helps reduce thermal degradation, reduce chain scissions and enhance optical clarity. This is a moot point for painting applications as one would not be processing the plastic at or near Tm.
ENHANCED UV RESISTANCE
Some plastics are inherently more UV stable than others. Polyimides are very stable. Fluoropolymers such as Teflon and PVDF are stable. PET is fair. Polycarbonate, Acetal and ABS are poor. With UV absorbers and UV blockers added one can improve the performance and such improvement is most appreciated when PET is used in thin film solar cells considering the temperature, optical clarity and dimensional stability requirements of these cells. Your guess is as good as mine on whether they are useful in painting substrates.
Thermoplastics are susceptible to specific solvent attack, but propylene glycol is not harsh enough to do any harm to PET.
With respect to adhesion, there are two distinct schools of thought. One emphasizes surface energy and the other amorphisation.
The surface energy school believes that flame, plasma, corona, abrading treatment etc. change the surface energy promoting adhesion in difficult to bond substrates as proved by water contact angle measurements or dyne pens. Bad wetting leads to poor adhesion and so good wetting must lead to good adhesion (?).
The amorphisation school believes that the surface energy effects are too small to be of any relevance to bonding, but flame, plasma and corona treatments make the semi crystalline polymers like PET more amorphous and coatings are able to intermingle and entangle with amorphous surface that have much less order than crystalline surface.
No matter what school you believe in, choosing amorphous PET (APET) or PETG over crystalline PET along with a bit of surface abrading and cleaning should put you in a winning spot.
Dr. Vaikunt Raghavan