Andrew, I leave the gelatin coating -- and I have noticed no difference between coating the image or non-image side. I have a few sheets of litho film that I hav e used 20 times or more -- but eventually the original gelatin coating starts to flake off (not surprising considering the number of times it gets put into 120F water.)
Here in the States, acetone can easily be found in hardware stores...especially in painting/refinishing stores. I buy it by the gallon (3.8 liters?) -- lasts a long time seeing how I use 14 ml per 8x10 tissue.
Strange thing about Potassium dichromate (PD) and Ammonium dochro (AD). Some folks work with PD and Iso with no problem as a one-shot tray sensitizer...and I have had bad reactions between AD and Iso (the solution would start to quickly darken). it worked fine when I first started to carbon print -- perhaps the later Iso had additives. I switched to acetone due to that...and much faster evaporation times.
Venchka -- no upper limit. After the tissues seem to be dry, one can put them in a box and/or plastic bag to keep them stable. Too dry, and the tissues start to curl excessively. One can modify the amount of sugar and/or gylcerin in drier climates to help the tissue retain enough moisture to prevent excess curling. Too much sugar and/or gylcerin can cause the tissue to always retain too much water -- risking damaged negatives when printing...and possibly poor transfers.
In my relatively cool damp climate, I actually use more sugar than most (but little or no glycerin). I use 80 grams sugar per 1000 ml of "glop". One just has to experiment and see what works best for one's working conditions and work flow. One size does not fit all with this process! But I think Andrew's receipe is a good starting point. I use much less pigment of a different type (about 4 to 5 grams of lampblack watercolor paint from tubes for 1000ml of glop).
If you really want to sponge up carbon information -- check out...