How does rust start in obscure places?

[nehuge]

Sort of a newb "in theory" question, but now does rust really start? I get the whole thing with salt roads on the undercarriage where rock chips can come up and compromise the coating, or the front fender arches get chipped up, etc.
But how do we get the obscure locations showing up like in these pics? I mean maybe road "spray" with salt in it can land in places on the car like this window sill, (and in my case the top of the rear sedan back window) but with no chips or anything in the paint how does it start? Even if salty water was pooled up in the middle of the hood with no chips, how does it go through the paint to begin with? This is what bewilders me. Especially on the side rear windows of the wagons too. If it is just a rubber seal again a painted surface, how does it start to eat through it without metal exposed? When the water got into my rear window and ended up going down the c pillar and down the rear wheel hump and pooling on the rear floor pan, aren't the floor pans treated and at least painted? How does it go through that paint?

 

[luckymike]

I'm guessing there's a metal clip holding that strip on. The barb of the clip breaks through the paint…

 

[WDB748372]

Prior damage to the panel or poor body work could also be a reason without any salt involved? I found out my Jetta had been in an accident when quarter panel began rusting and in an attempt to make it last a bit longer I noticed there was body filler.

Another way I had rust start is just behind the front wheels, on the rocker panel. Obviously by the time I see it outside it's already too late but debris/leaves being kicked up plus driving in the rain, all of that junk (a grocery bag full!) was sitting there wet for who knows how long.

 

[500AMM]

Sort of a newb "in theory" question, but now does rust really start? I get the whole thing with salt roads on the undercarriage where rock chips can come up and compromise the coating, or the front fender arches get chipped up, etc.
But how do we get the obscure locations showing up like in these pics? I mean maybe road "spray" with salt in it can land in places on the car like this window sill, (and in my case the top of the rear sedan back window) but with no chips or anything in the paint how does it start? Even if salty water was pooled up in the middle of the hood with no chips, how does it go through the paint to begin with? This is what bewilders me. Especially on the side rear windows of the wagons too. If it is just a rubber seal again a painted surface, how does it start to eat through it without metal exposed? When the water got into my rear window and ended up going down the c pillar and down the rear wheel hump and pooling on the rear floor pan, aren't the floor pans treated and at least painted? How does it go through that paint?

To my understanding it is two main factors:
1. the coat adhesion to the surface - which depends on both the coating process parameters and the specific coating properties
2. automotive coating has porosity i.e. they are capillar, which means fluid molecules may penetrate (transport through) over time

Read & enjoy:

automotive paint porosity - Google Search