@Stefan Lochbrunner and @esot.eric were thinking about using a dvd laser to ablate spraypaint for making PCBs as seen <here>. I had a laser working, but whether I tried paint, marker or tape, it just didn't work right when the stuff was stuck to copper.
Taking apart a few laser printers, I struck on the idea of using toner - it's a plastic powder that melts on, and is what people have used for making PCBs at home.
I tried dusting on the toner powder, but that failed epically. But I did eventually find a method that works!
1) Mix some toner in with some alcohol (water doesn't work due to surface tension and this dries faster anyway). It shouldn't be pasty, but you do want it quite thick.
2) Clean some copper clad board with steel wool or whatever, then pour on some of the toner mixture. Tilt the board to get full coverage, then shake off the excess. Watch someone doing tintype photography to see a professional at getting the constant thickness. As you're shaking it off, the stuff left on the board should already be drying. Give it an extra blow or two and you're ready to laser!
3) Use a DVD laser (mine is in an XY thingee outlined in other posts) to burn out the pattern of the traces. You may have to experiment with different burn speeds, focuses (its good when not quite focused) and coating thickness (varied by adding toner the the slop) to find what works.
4) Carefully use cotton wool or a soft brush to clear away the non-melted toner, leaving the melted toner where you want.
4b) Heat the remaining toner and the board above some sort of flame to fix it thoroughly in place.
5) Etch away the excess copper and you should (theoretically) have a board! Yaay :)
Here's a test trace (not etched yet). I could scratch it off easily, but careful handling should help (edit - problem solved by 4b).
I'm leaving on holiday but just had to try properly. The video files are massive, and I don't have time to edit/upload yet. Here's a money shot of the results though:
I added a step between 4 and 5 above, which makes handling the board without the traces lifting much easier. In fact, they survived harsh scrubbing without a single break. So this is good to know if it is going to be a silkscreen method.
I confess to speeding up the etching process with some VERY strong HCL, but don't even think of trying that without gloves+goggles+safety tie. Seriously, I happily splashed the H2O2 in my eye and bleached my fingers a little with it, and people normally freak out about the safety of that. So when I freak out about this acid, you know it's bad.
I am amazed at the tiny track width this thing manages. And the clearance between lines it can achieve! It looks like there are some broken patches in the photo, but these are left over toner.
Even though this was just a fake pcb sketched in inkscape in 5 mins, I am pleased with how this is looking and excited to try some actual boards in January, plus show the whole process in a video.