Spray paint generally sucks, it doesn't set well, it pills, and it cracks.
What you need to make shirts, is a good knowlegde of silk screening. Get some Speedball Ink and use that. If you're lazy get some fabric markers - I reccomend Rub-A-Dub.
But if you want "paint" for making shirts, get Speedball Ink.
"For we, when we, evaporate; oh, we breathe ourselves out and away."
-Rainer Maria Rilke