Just signed up to say thanks for this idea. I didn't quite use it as planned but it really helped me.
I bought a sheet of PEI which I just couldn't get success with. It worked great for the first two weeks, then the shine went away and none of my parts would stick, even a 20mm PLA hollow calibration cube would warp up. Wiping with 99% isopropyl didn't make a difference. Hand sanding with 1200 grit then 400 grit didn't make a difference.
I'd given up and gone back to BuildTak then I saw this thread. At the same time I read there are fake PEI sheets getting around which are a dyed polycarbonate or resin blend, apparently it's difficult to get the raw material which Sabic use to make Ultem 1000, so suppliers just get any old thing and call it PEI. Reading the behaviours described in this thread, what I have sounds almost exactly like polycarbonate.
I applied the orbital sander to my sheet at 80 grit. It got a "furry" surface but parts stuck, too well, I could barely get PLA prints off. I then sanded at 240 grit which allowed warping again. Sanding at 120 grit proved to be the sweet spot. PLA sticks and removes effortlessly. PETG stays down yet pops off with minimal force, sometimes even pops off on its own as it cools. As stated earlier it does need a re-sand when it gets smooth after a few weeks but that's no problem. I'm still using bed heat but otherwise this is the best print surface I've found yet. If only this PEI sheet I found wasn't so expensive.
I've also got a 2mm of Bayer extruded makrolon (their general purpose polycarbonate) which I've wet-sanded at 80 grit. I think that's the same material as featured in the first post here. I'm going to finish up a project I'm printing now then experiment with that makrolon sheet without bed heat. I've read the thread above and I'm trying to remove as many variables from Whitewolf's successful results. I don't have 60 grit orbital sander discs, only 40 or 80. I don't have bed probing to make the grid measurements and images (I manually level and I'm a peasant still running Marlin on 8-bit) but I'll clamp it to my glass plate and give it a try.
Anyhow, even if the PC doesn't work for me in future, sanding this sheet of whatever-I-have worked really well so thanks again!