Every year just before the winter, i power wash the TT chassis to get all the crud off. Then I mix 1/2 gallon of Marvel Mysterhy Oil and 1/2 gallon of WD40 in a spray bottle.
I liberally spray everything underneath. My last TT was 10 years old before I sold it - the chassis looked almost brand new. My current TT shows no signs of any surface rust.
I do the same to a 54 year old classic car i own - underneath looks totally rust free. My motorcycles get the same treatment (just watch out for the brake discs!!)