I noticed this when detailing the inside of my car! My buddy says he's certain the door is rusting from the inside but I had it rustproofed... If this is surface rust, I will sand it with 40 grit and paint over it but if it's inside, I don't want to sand a whole into my door. I also noticed a kind of plastic sticker under the door as wheel as some yellowing.

 

Here's a pic of the yellowed plastic. I'm not the original owner so this might have been installed by a previous owner.

 

The plastic just seems stained, and it seems to be a plastic protector of some sorts. A clear film bra, possibly.

The rust, as much as I'd love to tell you it's on the surface...but I don't think bubbling is a sign of surface rust. Sand that part down and get the rust out of there (hopefully it's just that little spot) and have it painted with touch-up or something.

I had rusting on the top edges of my front passenger and driver doors, sanded it down, cleaned all the rust I could see and painted it over with touch up. It wasn't a noticeable edge anyways, thank god. Its been two years and I have yet to see signs of it popping up anywhere.

You're lucky the rust is on the inside of the door jamb, not noticeable at all no matter how wrong you go on it, lol.

 

Thanks for the suggestion. I'll first use some rust converter but do you think I need primer? I don't usually put primer before touch ups but this is more severe.

 

If you want to go the full nine yards. I didn't bother with primer on such a small area, but it certainly wouldn't hurt and would most likely let your touch up paint adhere better and last longer. Then again, this is the door jamb, not much happens in there! So you would be fine without primer in my opinion.