Elmer's Glue on Canvas, then paint the whole thing. I have totally done this, but with a hot glue gun. Elmer's Glue seems to be a better idea since hot glue leaves behind strings and is harder to work with.
Draw a grid of squares on a blank canvas, then divide each into six smaller triangles. Mask the edges of all the triangles to be painted in the first color with low-tack tape, then paint. Leave to dry, peel away the tape, and repeat until al