wow, this thread is getting messy. everyone's talking about different things and nobody seems to know what everyone else is talking about :lol:
yes, toner transfer is often done on paper, however it is much better when you use press-n-peel, which is transparency with an easily peelable blue coating on one side.
With photo paper toner transfer, I often had trouble with 12 mil traces, and stuck with 16 mil traces whenever possible. I also almost always had broken traces after transfer, which had to be repaired with a sharpie before etching.
With press-n-peel, I have NO PROBLEM achieving 10 mil traces, every single time, and have never had a single broken trace. The makers claim it will work for 5 mil traces. I don't doubt that it's possible.
With the photo exposure method, I know people can do 10 mil traces as well, and I'm sure smaller traces are also possible. this method seems to do a slightly better job keeping all the edges looking "crisp" when getting into intricate, small patterns.
however, let me say this:
when you start venturing below 10 mil traces, your etching process becomes very important, to minimize etching under the sides of the resist pattern which would quickly eat such a fine trace... and this is of course independent of the method with which you put the resist on the board. For hobbyist work, I can't see much reason to go below 10 mil traces except possibly in isolated areas where you absolutely have to cram TWO signals between a couple of IC pads instead of one :lol: