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Originally Posted by Mike D
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.
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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.