ok imran....we had this discussion if you remember.
WD40 is not a rust remover...its a lubricant and will only prevent moisture build-up (which ultimately leads to rust) in small areas or parts...like nuts, bolts, screws, small joints, cracks etc.
for a car to be properly rust proofed...it needs to be sprayed with an oil or an oil based solution film that reaches all areas...specially underneath the body, subframe, chassy, joints, cravaces and the like.
so this is not really a DIY job...u have to get it done professionaly..the car needs to go high on a hoist to access the underneath.