Why does painting over existing rust not prevent further rust formation?

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mrspeedybob
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A properly prepared and painted piece of steel can go for decades without the slightest bit of rust. If however you start with a piece that has started to rust and paint over the rust, it does not last more then a year or two at best before the rust is causing damage again. Why is this?

The principle behind painting, as I understand it, is to separatete the iron from oxygen so that iron-oxide cannot form. If I put this paint barrier over pre-existing rust, why does it not prevent further rust formation?
 
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the most important ingredient in paint is the the binder (polymer) which does not allow oxygen reach the metal substrate.
i am not sure but,
i think the rust present on the metal is affecting the binder of the paint in some way or the other which is exposing the metal to atmosphere.
 
If the paint layer has no pores or cracks oxidizing should stop since oxygen and humidity are needed for the reaction to proceed. However, usually rust is not very compact and breaks easily, thus weakening also the top paint layer, which eventually can also break and allow again oxygen and humidity get in contact with the iron below.
 
As an automobile hobbyist, preventing rust is one of our number one priorities. We use products such as por-15, which is a moisture curing polyurethane. This encapsulates rust and prevents further spread.

If you are looking for a "paint" that will prevent rust, it will be a good start. If my memory serves me well the original purpose of this coating was to coat bridges, or seal concrete. It's a respected rust control product in our hobby.

Kurt
 
por-15 is good , if u have clean and dry surface where u spray it. On already rusted surface , i don't recommend it. Might make a difference if u r living in a country where temperature stays always on warm side , but at least here in north its not good. Most of enthusians ( car-releted 1s..) use stuff called acid-paint , on a bottom layer. Dont remember its marketing name , but it react whit rust , and after drying , makes solid shiny surface. Then u can put por or any other antirust paint on a top of it. Ofcourse,,before doing anything , grind the most of the rust away 1st. :)