I used Dintrol 3125 and it appears to soak into the rusy areas well. Can't vouch for the long term effect. I also tried waxoil but didn't like it, is is to foamy and does not apear to soak in.
If your carpet is loose you can get at the sills from inside. Look at this pic from Indyz12's resto project.
http://forums.tipec.net/download/file.php?id=3080You can see rubber plugs in the picture. You can pull them out and spray into them. Need to do this myself before winter.
Another place worth spraying is in the boot. I notice the metal at the light clusters rusts. Also if you pull out the spare wheel there are more of the plugs like in the picture above. There is a cavity there and if it rusts you get rust from the inside out just below the number plate on the rear.
Mike