I read in the manual that "most parts of your car has been treated to protect it against rust," this is not verbatim, just a paraphrase. Now I'm in New England area, will be facing winter soon enough, with all its road salts. I don't know if I should have the bottom go through a "rustproofing treatment" that the dealer or detailer does. But I don't want to pay for it if my WRX undercarriage is already "rustproofed" in its stock form. Anyone have more knowledge about this?

Also, someone enlighten me about what rustproofing does. Does it mean you won't have to flush your undercarriage again, or at least for this winter/2 winters, etc.?

the undersides are painted but rustproofing/undercoating never hurt. Plus undercoating acts as a sound deadener.

Jeremy