Same thing here, it is caused by rocks hitting the lens and making very small cracks in there and then the water gets inside and "fogs" it up. I dont see how acetone or toothpaste on the outside will get to the inside and clear it up, u just removed grim build up on the outside of the lens. New lens cost $375 each from dealer (kyoto lens) $180 each after market and fine some on ebay for about $220 a pair, (must ask seller if they aftermarket or OEM)