I jack the front of the car up every 2 years and spray warm cavity wax into the grot trap. Leave for a day or two until it oozes out of the rear of the chassis. I use Dinitrol 3125 and a Pro Wax kit from rust busters,( which has sadly doubled in price in the last few years). I do not use my car in the winter as I have a Morris van, 2cv and Mazda bongo that are used all year round. They are all rusty piles of carp now.....