3G Opaque Heat Press issues

Hi all,

I'm a newbie here (for posting, been here reading for a while now) and have a question. I am trying to apply a color wash print using Neenah 3G Opaque onto a black poly/cotton (actually an apron, not a t-shirt so heavier fabric) and I'm getting a pebbly or uneven surface. I have the heat press set to 350F at 30sec with Medium pressure.

Wondering if it could be a combo of things causing this...
Pressing too long
Apron surface is heavier and just causes more "uneveness"?
Needs More pressure?

Any thoughts would be appreciated. Also I know this is for t-shirts, and this is not the same fabric but I wasn't sure exactly who to ask this question of, so if there is a better venue, open to that suggestion as well.

Thanks