PCBLaserEtch

laser-etched chemical etchant mask for DIY PCBs

Clean the copper surface if dirty using a scotchbrite pad
Spray paint with a flat black spray enamel, one or two powdery (solid, not drippy) coats just so that the copper isn't visible anymore. Do not use glossy paint, it tends to curl under heat.
To keep the etching process efficient, design your circuit with as wide traces as possible, and with as much copper as possible (i.e. use a "ground plane" or draw black squares around everything so there's less exposed copper for the acid to etch away.)
Export your file as a black and white bitmap image with 254DPI (100 dots per centimeter, the laser is metric). You might need to modify it in a graphics program after export.
Place in the laser cutter and engrave your pattern (MIRRORED, so your circuit is on the backside) into the paint.
Remove from laser and dunk into FeCl but do not agitate (or do anything) for 3 to 5 minutes.
Remove board from etchant and rinse with water gently. The paint soot will be more loose and the paint will have hardened
Lightly wash the soot from the board using a Q-tip and some 91% rubbing alcohol. Be careful of small features, and be thorough
Place back into etchant and etch as usualy. Any problem areas (soot you didn't get) will have to be touched up after a quick rinse with water with the FeCl on a Q-tip after removing the soot with alcohol.
Remove spray paint with acetone

Done!