My 67 210 has never had the body off. During wy restoration Rust was worse in the areas of heavy factory undercoating. Front fenders and rear quarters. I agree let the water drip dry.During the restoration it was decided to paint the inside of the fenders as if they were outside. Two primer coats and two single stage enamel.I did not undercoat. This way I can watch for any signs of rust.The underside of my 210 body has surface rust.We may sanblast it and primer. or wire brush and put on some kind of brush on paint. Im not sure what to do yet. I think these cars would have weathered the 40 years better if undercoating was not used. It does help to sound proof when rocks get thrown up under the car. I wonder if the 210 in the volvo musuem has any rust?Good luck and enjoy your great Volvo.