Originally Posted by MDCTFTW
I wouldn't use t shirt material. You don't see many detailers using t shirts to wax/wash cars for a reason.
It's significantly better than sawing at your paint with fishing line. And there's no reason a microfiber wouldn't work the same way.
That said, a nice, soft, cotton undershirt is not going to hurt your paint in any way, shape or form. It didn't damage my paint at all. However, I did give the whole area a quick buff after removing the stock spoiler and adhesive since it is normally difficult to get to.