THIS IS VERY TRUE! I left the bra on my toyota and it ruined the paint underneath. When I took it off after one day, the paint was different and splotchy, and yes....the dirt under there, even though it was just a little dirt, rubbed the paint. I had it on about a year and didn't think anything about it. I eventually sold it with the bra on it.
Don't use the bra, use touchup paint and wax often and hope for the best.