rust never sleeps
 

I bought an '84 300TD about 6 weeks ago and there is some rust I'd like to take care of ASAP. There is some rust under the rear passenger side window and...sigh...I just dinged the gate and scraped the paint. I figure rust is like an iceberg, what you can see is only the beginning, and I'd like some advice on how to find out how deep it goes and what I should do once I find out. Also, what's the best way to treat my new ding?

Thanks in advance for your help

My guess is on the tailgate it is oil, not rust that is making those spots. Could simply be a leak where the spray makes it to the back of the car. Do you have a transmission pan leaking?

I had a father-in-law who sprayed his trucks (lived close to the ocean) every year with diesel.

Carefully take a close look at all areas of the car. Sometimes it simply isn't worth it chase some rust.

A quick and simple way to slow it all down is to wash the car with warm water, dry completely and then spray areas with WD40.

Don't go to car washes where they proudly claim to recycle their water, especially in the winter. You are getting salt baths each time you do.

Haasman

The window area rust can be treated by removing the aluminum trim and carefully wire-brushing and sanding down the rust. Unfortunately, you'll inevitably end up sanding some of the paint in the adjacent area, so the finishing (prime and spray touch-up) will probably show up. The difficulty is that there is some thickness between the level of bare metal and the finish paint, likely clearcoat, and it's quite a job to build it up again for an invisible touch-up. This is minimized by feathering the adjacent painted area and building up and blending the primer before finish coating.
I have no idea what your skill level is, or what you want to spend, or what level of finish quality you seek. At the lowest level of all three, you could treat the rust with some chemical and tough up paint. IMO, that will look like s**t around the window trim. You could carefully sand the exposed rust and paint, without removing the trim, but you'll be back again in less than a year. If a pro is to be hired to do the finishing, removing the trim yourself and sanding off all the rust and priming the area will save big bucks.
The ding looks like its just down to the primer. I'd remove the interior panel to gain access to the back of the metal where the dent is. Take it to a ding-doctor if you haven't the skill to DIY. If you can do the work, I'd duct tape a flat dolly and use a body hammer to flatten the ding, from the inside, then spray touch-up the area.
For small areas like these, an air brush works well...but you still need touchup paint, clearcoat, and primer.