I found the masking tape and leaving the white space the most effective.  My ancient bottle of masing fluid did ok also.  I'm going to blame my paper and the fact that I let my paint dry to long as to the failure in removing paint with a wet brush and with a sharp object.  I've never had success with Gouache, but that could well be due to the fact that the gouache I have is over 20 years old.  Great class.  Great teacher.