Don't worry about the weave. Twill or plain weave don't change strength. If it was twill, it would cost a bit more than plain weave, but it would drape the rails easier too.
My guess is it was how you squeegeed the epoxy in. Epoxy wants to soak in, nor push in, so you have to spread it out, let it soak, then lightly remove the extra. I said lightly, because if you scrape too hard, the cloth flashes like that. Basically, the cloth may gave been scraped too dry.