Do you mean under the paint? If so, yes I have that. When I was painting my tank last summer I Wet sanded deep enough to hit all the bondo and body filler. I washed it, put it in the sun for a few hours and started painting!
When these "warts and blisters" started popping up all over the tank, I posted some pictures. A few members on here explained that you can not get body filler wet. It holds moisture. It gets out later.
I popped a few of them and they turn yellow on the white. If you don't they stay about the same size.
