Sorry for necroing an old thread, but this was the inspiration to try and ditch the methods involving laser printers, so I thought I'd report on a success, sort of...
The msla printer I used was a new out of the box Elegoo Saturn 4 Ultra (because it had the high resolution 10" mono lcd, and it has a tilting feature that supposedly makes 3d printing a lot faster, though there are bugs with the whole design), and this is most definitely not an ad, I do NOT recommend it for this PCB exposure thing, because there is no easy way to just pop your image on the screen for certain amount of time, so get around this I you'd need UVTools freeware software, it is used for creating and editing 3d print files, it will open .goo (Elegoo format) files that you can feed from USB into the printer, so I took the example file that came on the USB together with the printer, deleted all layers, imported new layer using custom .png image (had to be resized, because pixel dimensions are 24x19 micron, they aren't square), then edit the layer settings, set exposure time and hit print, aaaand wait for it to go through all the pointless (for this purpose) actions, calibrating some mystical parameters etc, and then override couple errors to get your image to pop up on lcd finally.
I initially tried couple dry films from Ali, the really cheap ones, and they are no good, it seems they are too thick and overall they have defects, like wrinkles in the dry gel that won't adhere properly, and the thickness seems to cause problems exposing narrow features, after exposing and trying to dissolve (develop) the resist, I can actually see crisp top layer of the features seen below (that is just a test image of TSSOP 24 with 0.1mm track weaving in between the pads), but the thin parts in between seem to catch some UV and partially cure also, making the developer a bit more concentrated kind of helped to dissolve some of it, but it also started to lift some other features, basically - a fail.
I tried all sorts of things with the UV intensity settings, exposure time, but nothing really solved the above, the film I bought needed very short exposure, only 10 seconds, going lower started to look like underexposure, developer was kind of starting to attack the exposed resist.
this is what it looked like overetched trying to get the pads to separate
And this is the result with spray on positive resist called Positiv 20, manufactured by Kontakt Chemie, this was first time trying to use wet resist, made a spin fixture out from a pc fan with some double sided sticky tape to hold the pcb, sprayed the resist on, just a thin coat, it doesn't really spray that evenly, spun it for maybe 10-15 seconds, that leveled it reasonably well, baked for 30min at 60C.
Exposure settings - 45sec, etching with plain ferric chloride, slightly warm, maybe 40-45C, etch time was probably 6-8 minutes, face down, constantly moving it
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