Probably the biggest issue with these types of printers is how the underside of the print will end up with very poor quality. The print gets submerged in the resin and the light source will extend through the currently printing layer and cure resin on the underside. There’s no way to stop the light source from doing that, the really expensive printers get around this issue by using a print head that places the resin where it needs to be cured so it’s not covered in resin in the first place.
What I’m thinking is that for each layer it spreads a thin layer of resin on the bottom of the tray instead of submerging the print into the resin each time. By reducing the amount of resin that it goes into, it should improve the results.
On a more technical side, there’s some things to consider–some liquid resin will still be on the print–anything that sticks to the edges of the layer and on the underside and top, but the amount that’s there should be much less than it currently would be. Plus, you can consider how oxygen prevents the resin from curing, which should mean that some resin stuck on the outside of the print shouldn’t cure.
The method could be done possibly mechanically without adding another motor—you could use a gate to hold back the resin and use a slot mechanism to move it up and down while the tray slides from side to side. The gate would move down to clear off the bottom of the tray and then when the tray moves back it would move up slightly to leave the thin layer of resin. The tray would have to be a bit bigger though, so that the gate could hold the resin to one side.