Background story: I bought a 3'x5' cotton flag and I painted it as seen in the photo. I made a mockup in photoshop and used a big T-square and pencil to sketch the shape of the numbers. They turned out super parallel, very straight, and very accurately sized.
I covered the edges with painters tape and filled in the tricky white areas with painters tape also. I taped down a trash bag on the larger white areas to save time and lots of tape. I then sprayed the flag with spray paint purposed for fabrics. After I peeled off the tape I was disappointed that there were spots I thought had been sealed but paint snuck underneath the tape leaving blue spots on the flag (which I couldn't get rid off). In other places, the tape apparently didn't stick too well to the fabric so paint bled past and ruined my straight lines.
In conclusion: I want to make another flag, but without making these same mistakes. What should I use as a better alternative to painters tape on the edges? Wax paper? I'm open to any ideas that may help me avoid this problem next time I paint.