I've used Frisket film to mask the cat and Moon. I also made a template out of 90# card stock to use as a guide when cutting around the Moon with my exacto knife. The rest of the masking is done with masking fluid. The tape 'holds' my horizan line.
I've been working on Mike's cat for the last 5 days. This is when I'm reminded of just how fickle watercolor can be. What worked well the first time isn't cooperating this time. I remembered to mask in the stars and the cat's whiskers this time and I still need to paint the shadows/reflections of the cattails in the water. Then I'll be ready to add the pastels. I thought I'd post an update and include images of the process I'm using to create this particular piece. Sorry for that blurry image in the panorama but there's no going back now. Mike, I sure hope you like the changes!