I can’t speak to whether you shouldn’t HAVE to… but you definitely OUGHT to. Because every single resin I have tried will gradually result in increasingly robust adhesion with accumulated print runs…
When I sand the Build plate, I can clearly see the areas where the resin is worked into the microscratches because the grey dust from sanding sticks to the trace resin and leaves a discolored surface. This discoloration correlates perfectly with the footprints of prior builds.
And my resin sticking to the platform problem disappears when I am careful to keep sanding until I can get those discolored areas to wash off clean.
The resin CAN get into the tiny surface scratches and it can definitely get cured inside those scratches and the first few layers Are over cured and compressed to ensure a bond.
ergo- the cause is not the optics, its the build up of traces of resin that do not pop off with the rest of the raft- but that fracture and remain stuck behind.
The resin is Not becoming more brittle… grey and clear and rigid resins are Already pretty damn brittle- its just that when they stick to the platform hard enough, trying to pry them loose snaps them rather than popping them loose. Durable and Tough do the same thing- but both of those tend to not snap as suddenly- when they stick to the platform , prying them loose will bend and crack them or cause them to tear, more than fracture.
I would point out that Formlabs Does list the build platform as a ‘consumable’ and about the only thing on it that I can see getting consumed is the fact that I have to sand a little bit off of it regularly to maintain usability.
I keep an orbital sander to hand with a pad big enough to cover the whole platform- with 200 wt sandpaper on it.
After getting the parts off, IF they are tending to stick more than normal, I clean the platform with IPA- dry it- hit it with the sander for about 15 seconds or so- then wash it again with IPA. If I still see the grey areas that look clogged, I sand a little more. Then clean with IPA