Just bought a used low mileage 2007 HHR, oil treated every year the body is clean and almost free of rust, but underneath the car, I don't know where the guy spent his time with it but it's full of dark dirt and there is indeed some rust. I am hesitant to remove all this. What should I do? Carwash and new oil treatment?

 

Engine shampoo , then car wash then oil spray

 

Not sure if anybody still does undercoating on cars and trucks.
They used to put them up on a lift and power wash the whole underbody before spray undercoating.
Do some calling around.
Ask each person you talk to for any suggestions.
When we were in Hayden Idaho the best car wash had you pay an extra $1 for an underbody fresh water spray to remove the road salt in the winter.
Not sure if it would remove heavy caked dirt ??

Silverfox

 

Possible that most of what looks like dirt, is actually undercoating, with a layer of dirt on it?

 

I'm seeing a lot of spray-on oil coating underneath places on the hot rod shows. That would certainly attract a load of dirt & get it very dark.

But that is 'supposed' to prevent rust. But the dirt might trap water & actually cause rust.

 

You don't show your actual location.
That could make a difference in whether you will be dealing with more salted and treated roads.

I saw a guy with a small pickup.. and his rig was jacked way up from the side so he could power wash 1/2 the underneath at a time.
It looked like he was a Mudder. It also looked as if he had done that before.
That might be a problem with the HHR and Jack lift points.

SF