I use wd-40 sometimes on my engine bay if I go off roading and will be doing water crossings and mud. Not a lot just a mist.
My whole frame/underbody is covered I. Fluid film. I used a brush and laid it on there.
The inside of the frame is painted with Eastwood Internal frame Coating. Then I covered it with Eastwood heavy duty grease. ( I actually really like this product. It's the same concept as fluid film. But when it dries. It's like glue! And it's thin. Look into it. I bought like 6 cans. And sprayed inside of the frame. And small body cavities)
Frame looks greasy. But I can assure you that it's RUST FREE AND WILL BE RUST FREE FOR A WHILE !