Years ago I had some rust start to form on my then new 1977 VW Bus.  I
applied a compound which was then pretty new, I think, called "Extend." This
stuff was promoted as being able to chemically combine with the rust and
stop it dead in it's tracks.  The literature said that there HAD TO BE RUST
in order for this stuff to work, so not to use it as a preventative.  It was
a water based stuff that you could apply with a brush.  I used it on those
spots where rust was beginning.  It turned the rust to black and killed it.
I kept that bus for years and then sold it to a friend who kept it for years
more.  This was in Florida.  Those rust spots never grew.  I don't know if
this product is still on the market and as I'm now mostly in the desert it
is not an issue with me.  But for those with the need, I HIGHLY recommend
the stuff.  At the time it was available at K-Mart and hardware stores
pretty commonly and was not expensive.  I think it was made by DURO Corp.

Scott