My old cars always develop rust spots. I try to deal with them before they get worse. But, to be honest, I am not good at getting a satisfactory finish.

I did my 85 300D a couple of weeks ago. It is white and quite forgiving. Now, I am working on our 98 E320. No serious rust, but 5 araes where there was bubbling under paint and one just above front bumber on driver side fender where there were perforations (looking inside, it was obvious why rust occurs there (a small ledge that collects salt & dirt.)

What I have done, is scrape areas where rust has bubbled through. Then sand those with course paper until I get out beyond the rust. (I ended up with 5 spots about 1-1.5" diameter and a strip above lower trim about 1/2x3")

I applied rust converter, let it dry and then washed with water.
Next, I painted spots with a POR15 type paint, using a brush.
Sanded that with 320 dry and then sprayed on a grey primer.
Sanded primer with 600 grit wet. Repeated this to try and fill small cavities
Using spray custom color matched at NAPA, I applied a couple of coats.
Probably need to clearcoat still (but need to ask Napa if the paint contains clearcoat)

Unfortunately, results not too good! Problem is the POR type paint. It dries so hard that it is difficult to sand smooth. End up with lumpy looking surface.

I need to find a high build primer that is available in spray cans.

I found this article on-line. Not bad considering source! https://www.readersdigest.ca/cars/maintenance/how-fix-rust-your-car/

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Try a 2K Urethane primer. You can get it in "rattle" cans.

As soon as you said you used a rust preventative paint I knew you were in trouble. Great stuff and I use it a lot but it has its place and exterior body work isn't it. As you noticed it's almost impossible to sand smooth and in fact, by sanding it you removed its rust preventative properties.

The rattle-can urethane 2K primer is available from Eastwood and other sources. It's pricey, $25 USD or so per can, so unless you've got a reasonably large area to cover, not exactly cost effective. Once you mix it you've only got a day to use it before the mixture hardens inside the can.

https://www.youtube.com/watch?time_continue=54&v=JeeWzI2iZZk

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“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

You are right about the POR type paint. The one I use is called DOM16. Similar, but slightly different when being applied.

I try to put on a thin enough coat that there is still space to add conventional primer and finish paint over it. But on vertical surfaces, there are always sags and that means sanding the high spots. Hard to do.

Sanding high spots shouldn't change the POR/DOM rust prevention properties. The paint really just provides a barrier to prevent moisture getting to the steel. The layer nearest the surface is not 100% cured, so will trap any moisture that gets that far (it is a moisture cure paint) Sand the whole surface too much and you are right, it will cure all the way through and only act as a physical barrier.

Nevertheless, this is the last time I will use POR/DOM on exterior bodywork. In future I will probably first do the rust converter. Then an epoxy seal coat. I have several types of marine expoxies. May thicken a little and also use it as the filler. It can be sanded.

Assuming I now have a smooth surface, I next need to learn how to better blend in the new paint. Even although color is supposedly same, a "cloud" can always be seen around the repair.

Our 98 E320 is still in pretty good shape. But it is not worth much. The drive train has never needed any work. Paint is original and overall looks fine. Just small rust spots showing up each year that I try and fix. Not worth spending more than car is worth on a complete paint job. Mind you, I did that about 9 years ago on my 85 300D . But it needed it.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5