Homebrew PCBs

When you expose board with mask the ink harden. But if mask isn't full 
black (the laser print isn't full black) then there is practice to not 
expose too long time as unwanted area may be exposed too. So you just 
keep the swet spot when difference exposed/unexposed is big enought to 
wash the unwanted ink off. Then you expose developed board again to 
fully harden the ink.