This is what I was secretly wanting from my Polaroid shots, blurry(-sh) colour splashed images the sort of thing the Impossible Project are doing but much MUCH cheaper.
Anyway, the process is simple:
1 – keep your negative backs
2 – get some bleach (thick-ish toilet bleach is enough)
3 – paintbrush
4 – tape, maybe duct tape because it’s strong
5 – a waterproof panel to tape the negatives to. I used, like the FFP and Filmwaster guys used, a glass panel from a picture frame.
Tape the Polaroid or Fujifilm negative black-back to the waterproof panel with the black side up and the ‘gunk’ side with the negative image facedown.
Dollop some bleach on the back and paint it over the black backing until it’s a nice sludgey mush then wash it off. Maybe two minutes of painting but it’s worth experimenting with.
Your image might deteriorate a bit but that’s part of the fun: