OH my goodness - I just read the link to the por15 rust stuff. I have never heard that the product is recommended to be just painted over rust or a non prepped surface. If that is even remotely true, whoever came out with that should be hanged drawn and quartered.

Nothing in the world will fix up rust besides cutting it out, stripping it back.

Once you have gotten it all out, phosphoric acid (which you can get from chemical supply houses in fairly decent quantities cheaply, or for about 50 times the price from a autobarn or whatever (it's variously sold as de-oxidene, rust buster, god knows what other brands) can help. It is not a magic wand, but best guess is that it reacts with the surface layer of metal and more or less 'hogs' the metal and doesn't let the oxygen react with it, forming a sort of boundary or surface barrier to the beginning of rust. You still of course want to follow this coating with prepping, proper surface primer then the epoxy paint (for the undercarriage at any rate).

Do not EVER paint over rust. Do not ever think you can get away with less than full removal of any rust.

Do NOT leave a car in primer any longer than absolutely necessary (it is porous and will let moisture through). You see cars for sale that have been in primer for months - nothing short of sandblasting the whole darn shell that was primered will do. You'll see rust come right through before you know it.

Another thing to be careful of is galvanic corrosion. If you put a ferrous metal and a non ferrous one in contact with each other they will start to corrode one another. This can occur occasionally when hardware store screws or bolts are combined with bits of plate (alloy or something). Probably the most common place i see it is on trailers - home made ones - with angle iron frames and alloy sheets for the floor or vertical bits.