I already knew that I should use a flat stable surface to iron on rhinestuds, rhinestones, and heat transfer vinyl (htv) with no steam. Earlier in the week I was testing out the ironing on of glitter vinyl and was using the cardboard that came with the packaging underneath the test fabric. I didn't notice what was happening until I tried this with regular HTV.....Notice the pattern in the hat.
That is because the cardboard I used to iron on was corrugated. Not a good idea. It's not a good idea because it does not provide even pressure. Even pressure is what you need when ironing on htv and rhinestones. I normally use the .022 chipboard in the inset of the photo below. You can use any hard surface that can withstand high heat...but never use anything soft or has air holes like the corrugated cardboard.
Hope that helps.