My experience is with Naval Jelly. I have a car that when I bought it ten years ago had a lot of heavy surface rust on the chassis (not a Porsche). No perforation but some fairly deep corrosion. I was living in a fairly remote location so my optiions were limited. I thoroughly degreased the chassis, removed any remaining paint (there was veryy little, and applied a very heavy coat of Naval Jelly. This comes in a jelly form as the name suggests and sticks to surfaces very well.
The instructions on the container said to rinse off after ten minutes-that seemed FAR too little time so I left it on for 24 hours and then pressure washed it off. The result was better than I could have hoped for: The chassis looked like it had come out of a dip tank. There was a whitish deposit left where the rust had been. There was NO evidence of rust remaining. I dried the chassis and sprayed on a good quality primer followed by two coats of Imron. Nearly ten years later the chassis still looks terrific and in the odd spots where I have knocked off paint with a jack, for instance, the whitish finish is still visible but no rust has ever reappeard. I think the 24 hour "pickling" period did the trick.

Gimme three steps...

I have used POR15 for years with great results on farm equipment and cars.

Three steps here: POR15, Inc. - Stop Rust Permanently - Repair Gas & Fuel Tanks

Only then do I paint over it.

Nothing comes close to this -

For great rust removal Safest Rust Remover

I have used this. I have recommended it. It works as advertised. The people I have recommended it to have raved about it.

I will NEVER use any other chemical rust treatment.

What you put over de-rusted metal is your choice. Eastwood's rust encapsulator works - it is coro-less repackaged under a different name, but it does not remove rust. Ospho and naval jelly convert rust, but do not remove it. Por-15 is tough, but does not remove anything. Safest rust remover and sandblasting remove rust. Where I cannot sandblast, I use chemicals. After I sandblast and want to be SURE, I still use the chemicals.

Por15 works great when applied correctly (following the steps above). The most important, usually unknown fact, is that there is no UV-inhibitor, and as such it must be top coated if it is going to see the sun.

Por-15 has a topcoat called tycoat. Its for uv protection. Look up a product called rust bullet. I used it on a plow truck i own and was quite impressed.

They have plenty of top coats, black coat and chassis coat are great as they come in spray cans now.

Never used any of the products mentioned.
Been using phosphoric acid to remove rust and convert what can't be removed with a die grinder and wire wheels for the last thirty years.
It has been the standard of that industry longer than most people here have been alive

Ospho is one brand I know of, and it's usally available by the gallon or quart bottle at Home Depot and other hardware stores.
Ospho Rust Treatment - Since 1947

Some old school guys even use coca cola if nothing else is around because it has a small amount of phosoric acid in to give it a little zing in the flavor along with the carbonation bubbles.
Thats why coke or pepsi is bad for the enamel layer on your teeth, it disolves and removes some every time you take a sip.

JFairman - what do you use on top of the reduced (former rust) layer?

Use Ospho, it is an acid that eats rust. Then, use primer and paint of your choice.