Through The Trees. The best stuff I've ever found for masking is this stuff here. The entire back is sticky so it stays put while you buff your background color on.
I have the Friend Jars/Fillers as well as the dies, but hadn't even used them yet. So I thought it'd be neat to make a gatefold card. I cut the Graphix sticky back transparency sheet using the Jar die and stick it right over the paint brush. Then I used the sentiment "You color my world" from the TT: Valentines set which I thought was a perfect fit for this stamp combo! I photographed this card at an angle so you can see the shiny transparency. Makes it look like a real jar, plus it was so easy just to stick that die cut on since its back is sticky. Looks like it came from Hobby Lobby (price tag), but I think I got it at a yard sale for like a quarter or something. Score!
Here is the card showing off the gatefold a little better.