Tests were done in which they examined the surface left behind after grinding or sanding rust and they found that there were pits left afterwards that they called seed rust. Once covered over the rust resumes from the inside.

The chemical treatments for rust neutralize the rust so that it is stopped. Rust is oxidation, the metal takes on oxygen and gives up hydrogen. The chemical rust treatments put the hydrogen back in the metal and force it to give up the oxygen. This is the jist of the idea and not meant to be an entirely accurate presentation.

One of the definitions of an acid is that it gives up hydrogen in the presence of water. If your rust treatment says to use water, you must use water to make the chemicals work. It is not watering-down the chemicals. The more is better mentality doesn't cut it here, use the water. Follow the directions.

Clean is good but mechanically removing the rust before chemical treatment may hinder the action of the chemicals, follow the directions.

A few thoughts on POR15.
First, the acrostic is " PAINT Over Rust"
I've used it, as well as some "similar" products. I agree with the person above who said he likes to wait many years before trusting any new "rust: products. There are many short-term helps, but very few long-term ones. You plan on keeping the car; you want long-term solutions.
Sandblasting is the best way to remove rust--It will get ALL the rust, and won't hurt the solid metal. There are some precautions to follow--but most are common sense---don't keep the nozzle aimed at the same spot for a long time; keep it moving. The "spot blasters" are OK, but you still need an air compressor.
Back on POR 15--I've used it over the last 15 or so years. Some of the vendors that sell at "Hershey ( Coming soon, Oct 6-9) bring samples of rusty metal they've painted. "Big Deal" I used to say.....until I saw the same samples year, after year, after year, after year, and they still look the same, after year, after year. Very Impressive.
I painted half of a rusty, worn out brake drum and placed it in a "moist room" I had available, and left it there for a month. No change on the painted side.
I'm not a vendor...just a satisfied customer.

Delano,

I just repaired a similar rust hole in the rear panel of my 115/300d. A local MB mechanic says that this problem is caused by a leaking washer around your radio antenna and a plugged trunk drain. He says that after you fix the rust damage spray the inside of the trunk well, not the entire trunk (although you can), with warm linseed oil. The oil will flow into the crevices and thicken. It doesn't harden and therefore remains pliable but not sticky.

Thanks. The pictures are a little misleading; the dash is black, and the exterior is dark gray. But in outdoors pictures, the exterior paint and the dash take on a blue cast. Or I suppose it could be my lack of photography skills

I'll be keeping up with this thread. Seems rust correction techniques aren't exactly cut and dried (ha ha, get it?).

Rather than linseed oil... which is sticky if not absorbed into wood or something... find some lanolin... sold in Tractor stores as Fluidfilm....
do a search here for more information on it....

Here are the pics of the rust- you can see it's underneath a thick coating of something. Paint or undercoating or whatever. I put some jelly on it for now, until my POR arrives. If you were to look topside in the trunk, you would see there is a seam in the sheetmetal traveling the length of the underside of the left rear quarterpanel. My Dad put some naval jelly and a piece of metal lengthwise across the seam, embedded in gutter sealer. These photos of taken of the exterior directly underneath. My concern is there is rust along the seam on both sides, under the paint/ undercoating. I just don't have the stomach to keep chipping more paint to see for sure. In one of the photos you can see the JB weld, that's where the recent repair was made. In that photo also, there are swell cracks in the undercoating which I have denoted and circled. I may have to mechanically remove everything along the underside of that quarterpanel, along the seam. Doh!

__________________
'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)