I'm experimenting with making prints with cropping masks. So far I've tried masking the negative with rubilith tape (which didn't block UV very well) and with sheets of film (which blocked the UV but left me with poor contact between the negative and the paper surface). I've also tried masking the paper before coating with mixed results.

Does anyone have any tips for masking negatives for UV exposure processes? Or has anyone got a reliable technique for masking the paper before coating?

Thanks in advance.