This is UV sensitive solder mask resin, applied as thin as possible using a silk screen mesh. Afterwards it’s heated at about 90C for 10 minutes. This makes it more sensitive to UV light by evaporating most of the solvent.

It is exposed with a 405nm laser at about 250mw of power. I intentionally unfocused the laser for a spot size of about 0.5nm

After exposure the pads are easily cleaned off with some IPA.