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On another forum a poster was using Castrol CL waterproof grease on the subframes sump etc of his Punto van, which was as new underneath after some 13 years.
My late cousin pumped and painted gallons of general chassis grease into the sills and cavities of an HB Viva he bought new, whilst it was filhty because dirt sticks to grease it was rust free for decades and is probably one of those few survivors.
Whilst CL grease appears to be discontinued, the above Smith and Allan 12.5kg tub of grease is meant for salt water application, unless you are prepared to go to the lengths some of the above posters (and respect given chaps, well done) have gone to, i might be inclined to steam clean the whole vehicle and once dry give it just a general wire brushing and paint where access is possible and then paint on a grease similar in properties to the one linked over the whole chassis, and use something decent that creeps to get into the joints and cavities.
The actually bodyframe area to concentrate on is the inner sills, between side steps and main chassis rails, if you look they get heavy weathering especially at both ends due to grit thrown up from the wheels, these can perforate if left and need treating inside too, which means aerosol or spraying via probes inserted via the various holes you can find via rubber bungs and if you remove the doorsill kick treads, these seections need treating twice a year i suggest.
Personally what i'm doing now, after going to town seriously with Bilt Hamber products when i first bought the cars, is every spring and autumn i get underneath and spray ACF50 onto and into the worse affected areas because it has good creep, this appears to be doing the trick as i can find no worse rust patching than when i first bought the vehicles, literally a half hour's job now twice a year.
We are paying the price of well heeled first owners who whilst keeping the body clean often completely neglected the undersides, never even giving a thought to washing the salt off underneath around April time, this year especially will have taken a serious toll on many vehicles, used during the winter but not being used during the spring rains which helps clean the salt off due to general house arrest of the nation.
I'm really anmoyed at Toyota for not galvanising the chassis and suspension mount/frames of their genuine off road designs, for the costs involved pre production they would long ago have put the opposition out of business, the general mechanicals are good for 40 years with some decent maintenance, its rust that kills them where salt is used extensively, any higher costs for new vehicle buyers being reflected in even higher residuals.