I have an 08' focus coupe that I want to get rust proofed. It already has surface rust on the underside of the vehicle, but does that making rust proofing useless if it's already there?

All vehicles will rust underneath in a very short amount of time. Heck, I bet some cars do this at the factory.

You said "underneath", but where exactly underneath? If it's on the floor pans, then that would be a concern. However a body shop can remove the old rust and put more undercoating underneath. I would suggest this due to your location.

If the rust is on suspension components, such as springs, shocks, tie rod ends, etc, then there is no reason to worry, as all of these parts will rust. My almost 20 year old car had a rusty suspension, but no kind of issue where safety would be of any concern.

From what I've seen, a "rust proofing" system that includes an oil based penetrant as a first step can help, covering rusty areas with a sealant alone can trap moisture and at BEST won't help....

Many of your countrymen I've talked to on the road still swear by the old trick of annual application of a "sticky" oil to the rust prone areas before winter. (Something that isn't "commercially" available here) I'm guessing that procedure is a "home cure" only, not commercially avail. anywhere...

POR-15 requires a bit of prep work for it to stick well, but if done right is great!

Or there are other rust "remover" and then clean the area (soda blast may be an option?) and then put a good coat of under-body coating on it.

Moved your thread to the General Car chat area since your just asking a question and not having a technical problem. [thumb]

move to the south

That could just make it worse... not as bad as the north or Midwest, but you would want to get away from both the salt and the humidity. Move to the south west to dry climate