After finishing your quilt you need to launder it. I use (cool/warm water ) and detergent. The markers come out when laundered.

Still, I stay away from red or black, and I don't iron over markings.

I've used them many times on all the quilts I've done. Only once did one not come out completely. I used Spray n Wash on it and then washed in the machine. Everything came out.