Homebrew_PCBs

You can find the stuff on EBay, UV Curable Solder Mask, but you won't find any more information than that you see here. The Chinese shops sell it but don't have any details on the stuff.

The step by step that James gives is essentially correct. The special plastic is nothing more than celohane and can be purchased by the roll from Walmart in the wrapping paper section. If you are using plastic transparencies you can dispense with the celophane layer and place it directly on the wet ink. I have not had to perform the second step of UV exposure of the entire unmasked board though. After a few minutes of exposure with your transparency mask in place, the ink under the transparency dries. Any board area with ink uncovered by the plastic will still appear to be tacky. For some reason the ink under the plastic overlay dries many times faster than uncovered ink.

This stuff works, BUT it is messy. And, it requires some trial and error to get the ink thickness just right and to get the exposure time just so. Different colors dry and expose at slightly different rates.