For what you're wanting to do, I'd suggest you get something from the Createx paint line, Golden Acrylics, LiquidTex, etc. You can reduce these, if you want, then yes, you can heatseal them with an iron. You'll want to iron the backside of the shirt, not the paint side and just follow the directions for time/temp (to fully seal/cure) onto your shirt. The Createx line is one that I've used for years in my airbrush work. The paint outlives the shirts if applied/cured correctly.