I use many different kinds of fabric paint when I paint and print on fabric, but I use ProChem Textile Paint when I'm printing construction fences, mainly because it's thicker and easier to control. The instructions for the ProChem Textile Paints say:
"Heat set fabric paint. After applying paint to fabric, air dry thoroughly and heat set using one of the four methods outlined. Iron at the hottest setting appropriate for your fabric from the back side for 5 minutes OR place a clean dry cloth over the front of the design and iron for 5 minutes. Do not use steam. Keep the iron moving to prevent scorching."
"Do not wash for 10 to 14 days after fabric is heat set. Wash fabric with luke warm water, gentle agitation and mild detergent. If patterned item is a garment, turn it inside out for washing."
Usually, when I have finished printing with the ProChem Textile Paint, I set the paint by ironing it for 1 minute when it's dry, and wash it the same day. This time I let the fabric dry, ironed it for the one minute the next day, and waited 2 days to wash it, only because I was going away for the weekend and didn't have time. A picture of the unwashed fabric is below:
I don't know if waiting the 2 days made a difference, or if I just printed the fabric darker than in the past and the paint was really in there, but there is very little difference between the unwashed and washed fabric below:
In the future, I'll try to wait a few days before washing if I can. I think darker is better than how light the fabric sometimes is after washing. I'll also try to document it again in the future, to see if waiting is really worth it.