craftgrrl | Speedball ink and messenger bag?

Everything I've read about Speedball screen printing ink says "NOT for use on nylon." Does anyone know why? I want to use it to stencil on a messenger bag because it's durable, but it's a polyester/nylon blend. If this is going to end up with disasterous results, what should I use? Is acrylic paint + textile medium durable enough?

I'm excited. :D I got a $5 men's shirt at Wal-Mart and two $15 hoodies (with embroidered logos I have to cut off) to stencil. One shirt and hoodie are for me, the other hoodie is for my boyfriend's brother. :D Right now Wal-Mart has thinner plain hoodies for $9, which would be awesome for stenciling! I tend to wear the crap out of my hoodies, though, so I opted for the thicker one.