gouache

I have a painting that I have done freehand, but now want to change it so that the lines are absolutely straight. I am using a new canvas and gouache. I have tried masking tape and painters tape, but the paint bleeds under both of those. Does anyone have any suggestions as to how I accomplish my...

Hey I am new to this forum and I wanted to share my Gouache Rob Ford portrait. Let me know what you think. http://morganskinnerart.com