Very simple, apply your used engine oil on the under carriage generously, in fact let it be dripping.

Then drive in a dusty place, the dust will stick to the oil and create a fluid film on the underneeat of the car.

Congratulations... You have now rust proof your car. Just endeavor to do it twice yearly you should be fine. Far better than using collta

dust is actually fine sand which has a good water retaining capacity. Rust is not only caused by water but by salts also.

What you have done is a breeding ground for microscopic organisms and bacteria, they would be glad to chop your frame small small till every metal turns into cardboard