It's not impossible. There is a guy that has a discharge blog on this forum, and he printed on some 50/50 shirts. The colors aren't as vibrant but it will work. The discharge just won't take to the synthetic fibers. I wouldn't think that 3% would be very noticeable.
It only works on natural fibres, but you can print on blends. If you print discharge on a 50/50 cotton poly blend the cotton will discharge, and the poly will remain its original colour. Could be okay, could be disastrous - depends what you're trying to accomplish. On a 97/3 blend, there's a good chance the 3 either wouldn't be noticeable, or would be noticeable only as flecks or occasional lines. The 97% discharged cotton will (hopefully) drown out the 3% undischarged lycra. You'd need to test to see whether or not you get the results you want, but in theory it'd be okay.
I do a lot of discharge and have the discharge blog mentioned earlier. I have done discharge on non-100% cotton in the range from 50/50 blend shirts to 90/10 hoodies to 95/5 cotton/lycra. If it's 80+% cotton, you'll get a decent discharge it just wont be as bright as a good 100% cotton discharge. 50/50 blends are very interesting, as you can get varied results, from a marbly textured looking gray if you're doing a 50/50 black, or a color half way between the shirt color and off-white if you are doing an orange or something like that. I've told customers curious in discharge about the 50/50 effect and some people have specifically wanted it.
Just experiment. That's what I do. Check out my blog.
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