Worth the use of filtered water for under the frame? No. The shop truck gets washed every time it goes out in the salt and the gmc gets washed nearly as often. Salt and rust occurs worst when it is allowed to sit and oxidize. Also I think it's important not to let anything build up. Dirt and grime will hold moisture on the metal. The only advantage to the filtered water is the prevention of unsightly waterspotting which is also damaging to older and single stage paints. Is it worth it to wash it all even underneath every day?? Absolutely!
Is it worth it to also wash the underside (the frame) to wash off the salt? Is it useless? Rust will eat the frame/suspension at the same rate?
Worth the use of filtered water for under the frame? No. The shop truck gets washed every time it goes out in the salt and the gmc gets washed nearly as often. Salt and rust occurs worst when it is allowed to sit and oxidize. Also I think it's important not to let anything build up. Dirt and grime will hold moisture on the metal. The only advantage to the filtered water is the prevention of unsightly waterspotting which is also damaging to older and single stage paints. Is it worth it to wash it all even underneath every day?? Absolutely!