Magnetic paint contains Iron Powder. also known as a form of ferrite. Ferrite is the reason Radar Absorbent Material absorbs radar signals. Maybe Magnetic paint can work in much the same way.

If memory serves me right, this has been posted before.

Radar absorbent materials do not totally absorb radar. They absorb some of it. The rest that isn't absorbed bounces right back to the reader.

There is no effective way to fool a radar gun, save doing the speed limit.