Agree with the Bonnet Bra advice after it rains whip it off and give it a hoover. 
Basically when it rains the water can carry grit/dirt down under the bra then as the bra vibrates and moves in the wind whilst your driving it acts like sandpaper 

Or do what I done last year
Wax car
Under the area where the Bra would go place masking tape (I used the blue 3m stuff)
Over Masking tape apply gaffer tape (This stops the masking tape from getting wet)
The apply Bra 
 
 
Left mine on for 3 months and not a mark in sight 

Ben