I am planning to back my polyester siderweb top with corduroy. Do I need a thin batting? I am worried that the brown of the corduroy will show through to the white polyester.

Maybe you could use a white flannel as the batting.

Maybe try a flannel or thin coton batting to see which doesn't allow the corduroy to show through? I would keep it thin though... unless you like really heavy quilts :D:D:D

You can get black battings in poly and/or cotton.
Also black flannelette.

I think I'd use either flannel or broadcloth (white) instead of batting. Anything else will really add to the weight.

Test a piece of the white on the brown and see in what sun or light the brown starts to show , that will give you more of an indication. If you don't want the warmth but just want to block the brown , use a white sheet in the middle.