If you find that you cannot keep water out of the backs of your sills, there are two places on the P6 that are always worth checking, especially if you intend to do any serious amount of welding to the sills and D-posts.

The first thing to do is to remove the rear wings on both sides of the car, and look very carefully (i.e. prod with a screwdriver) at the the whole of the inner wheelarch in line with the centre of the tyre tread. If that is OK, then look at the rubber seal channel that is spot-welded to the vertical face of the upper D-post, and look for any sign of perforation immediately above and behind the seal channel.

If the inner wing area looks good (and you are content that you have a P6 that falls into the minority group of no rot in this area), then the next thing is to remove the rear decker panel below the rear windscreen, gasp at the flora and fauna that has collected/grown underneath of it, and then look closely for any perforations in the horizontal panel near the outboard edges.

Most P6's that I have seen in scrapyards, over the 30 years that I have studied this marque, have failed in the rear of the sills and D-posts, and of those, all have perforation in one of the three places mentioned above. All of these places will channel water into the back of the inner sill area, sometimes so effectively as to brim over the internal vent holes into the car itself.

I hope to be able to create a photo-montage for the club of all of the favourite rust-points on the P6 cars soon, it's like a hobby to me.