Rusty stud/nub in rear wheel arch

[SurferUK]

On both sides this thing has broken through the rust-proofing. What is it and how would you go about stopping it getting worse and spreading to surrounding metal? For now I have just given it a squirt of ACF50.

(I have a pathological hatred of rust that only the 'Salt Belt' will understand...)

You can click on pic for a full size version, it was hard to get a decent photo.

 

[rdruss]

I would probe around it a little with a knife or pick to see if any underlying rust that is not obvious, if there is you can dig it out and seal with a glob of RTV or undercoat "tar".
Last year I discovered a situation in driver rear wheel well that had rusted through into the area above the side step. By the time I got finished digging out the rust the hole was 1-1/2-2" in diameter. I cut a piece of tin a little larger than the hole, drilled it for sheet metal screws, slathered it with RTV and screwed it in place over the hole then covered the whole area with more RTV.
It will probably need attention every year from now on.
I live in the rust belt so that is a never ending battle!

 

[SurferUK]

I would probe around it a little with a knife or pick to see if any underlying rust that is not obvious, if there is you can dig it out and seal with a glob of RTV or undercoat "tar".
Last year I discovered a situation in driver rear wheel well that had rusted through into the area above the side step. By the time I got finished digging out the rust the hole was 1-1/2-2" in diameter. I cut a piece of tin a little larger than the hole, drilled it for sheet metal screws, slathered it with RTV and screwed it in place over the hole then covered the whole area with more RTV.
It will probably need attention every year from now on.
I live in the rust belt so that is a never ending battle!

RTV is a good idea, I like your patching method too. I would have to do the same as I can't weld!