Rustproofing decision?

[91189]

I have an 89 cab. The undercarriage seems to be coated with cosmoline, as it does exhibit a yellowish color.

I would like to refresh the rustproofing with a good long lasting product.

I plan to pressure wash only, since I do not have the facilities to remove the cosmoline in a clean and environmently safe manner.

I am most concerned with protection rather than cosmetic looks, although both would be nice.

Can I spray Wurth Body Wax over the Cosmoline, as some say it is the same product?
Would Stone Guard adhere to Cosmoline, this would be nice especially in the wheel wells?
Is using grease an alternative especially in rust prone areas?
How about a combination of products?
What else is good?
What areas are more sensitive to rust?
What should I avoid coating except the obvious, engine and heat delivery system?

I would realy appreciate anyones experience

Paul