Users have experienced rust issues in cultus both on top/roof sides (minor) and bottom.
You must get good undercoating to prevent it specially if you reside in karachi (the rust and sewage city) .
Undercoating does not last forever so you must get it redone as the need be every few years!
In my opinion the factory paint quality and paint layer thickness is not good.
Others may differ and think of cultus as a diamond. All the power to them and my sincere happiness that you got a good piece from the factory.
Its just a so so car which gets the job done in its (over) price range. I prefer cultus over wagon r. Atleast its got a cabin AC filter!