rust treatment and POR15

Once you have a few holes in a panel you can be sure the back of it is at least as bad as the front. Likely worse as there will be less if any paint. And of course with sills etc you can't de-rust the back.

Also, where there has been rust penetration, the surrounding area is likely pitted, so difficult to get back to bare metal. Because if you grind the level down to the pits of rust you'll likely go through it.

If you can get all the rust off both sides, a decent acid etch primer is best.

When I was at college a classmate spent an entire year covering every panel of his 912 with braze. He used a carbon arc braze. What I don't know is how long that lasted but I often saw it around town in 5 different paint colours. He had gradually updated the front and rear from later 911 parts. It died over 20 years after the brazing when someone T boned him. Insurance paid out, then he sold the wreck for even more.

Pirelli student apprentices were paid far too much.

I've repaired a sunroof on the old Rover by lead loading. New ones are NLA. And most secondhand ones are rusty too. But obviously with the thing off you can get at both sides. I made sure there was no rust left - even used a magnifying glass. Be interesting to see how long it lasts.

Starters was the rust coming from the outer surface or the inner surface.

Maybe the inner surface rusting through? there is not much you can do except fish oil (etc) treatment for the sills.

If it was surface (outer rust) then you didn't treat the surface enough. its a phosphoric acid, etch primer then a the primer over the bare metal.

Primer is still porous and needs a top coat of paint to seal the job.

There are many things that will retain the moisture and these trap water initiating rust. (Body sealer)

On one of the Jags there were four rust spots on the sills. These were where four threaded studs were attached to the sills that were used to attach trim panels. It looks as if Jaguar didn't rust proof the sill properly after the weld. I had the rust cut out and new steel let in and the sills rust proofed. After six years I sold the car - still no sign of rust.

POR15 is great, right up the point a pin prick sized hole occurs in the coating and a tiny particle of salt enters. Then, over a period of time it peels off in sheets. It doesn't matter how good the surface is prepared, how good the key is, even new components overcoated with POR15 will succumb to total coating failure in a few years.

As alternatives, and depending on visibility of the area you need to protect:

Dinitrol RC800, overcoated with Dinitrol 3125 and then 4941. 3125 into the cavities. Sprayed with proper equipment that dispenses and atomises correctly.

Or derust and then use Galvafroid, followed by an appropriate topcoat

Realistically anything other than full panel replacement and a full body phosphate dip with wax injection into the cavities is a bodge.

I'm beginning to think I should have left it alone in the first place. It had been given a 3" line of black stone chip material and then a bottom half respray over the top when I bought it. This looked pretty nasty but was obviously keeping most of the rust at bay. I scraped it all off but obviously didn't make much of a job of it.