Homebrew PCBs

Hi all, I'm still working on the serigraphy emulsion system. I think I
need to increase the exposure  time to burn the emulsion a little
more. The problem is that I'm using transparencies with toner as
masters, and toner, even from a new cartridge, is not totally black.
So I went to the serigraphy shop and found a very interesting product:
toner darkener. It's actually called toner blackener in my language
(ennegrecedor de toner), but I found it as darkener on google. 
It's a spray, cheap, about $7 for the local brand and $15 for an
european brand, which the guy told me is used to make masters for
offset prints. So I bought that one and sprayed it on a transparency.
the effect was INSTANT, it dried up in a few seconds and the toner is
now totally black, even under the sun!
So to all people using photo resist, this might be an interesting
option. Also to people who use toner transfer, maybe this will help
with thin lines? I didn't see if it leaves residue as the test
transparency I used had no transparent edges.