I'm about to mount the windows in my freshly painted bodyshell and doors.
I have a bucket of Mike Sanders Rustpreventing Grease, fantastic stuff generally, and I was thinking to use it between the rubber and body. But I'm worried it might cause the rubber to swell or damage it in some way. Have googled it, but can't find any information about it.

What is the best thing to do, to prevent rust between the rubber and body?

I'm interested in this too. I think you're quite right, normal grease products will attack the rubber.

There is a product which is suitable for contact with rubber but I forgot what it is. I could do with some, so if anybody knows..

K Y Jelly

Is that petroleum based though? If not it's fine but it won't last long. I'm sure there's a proper rust proofer that's ok to use.

After my AK350 was resprayed in 2004, I brushed a thin coat of clear waxoyl on any rubber components that fitted against the body, (window rubbers, rear light rubbers, etc). 13 years / 27000km later there are no signs of the paint or the rubber being affected. However, that was all original quality rubber, (see other thread: http://www.international2cvfriends.com/forum/viewtopic.php?f=7&t=5977 ). Also I keep that one garaged most of the time (but for the first few years, starting with Kelso, it always seemed to p*ss it down with rain whenever I took it anywhere!)

Red rubber grease!
http://www.castrol.com/en_au/australia/ ... rease.html

Beat me to it, Roger.
Ah well, some Red River Valley as consolation...
https://www.youtube.com/watch?v=hKKoMTvTFOM

Lindsay Porter advocates using windscreen sealant i.e.: Mastic type product.

Hmm, hope I never have to replace another windscreen after that stuff has been used to install it; once was more than enough...

For what it's worth, I've always used Waxoyl when refitting window or windscreen glass on A series vehicles, with no noticable ill-effects on the rubber surrounds.

Thanks for the tip on the rubber grease! I was not aware of that product.
I will order some, seems to be perfect for window installs.

About the Waxyol.... What product exactly have been used on rubber? Waxyol is just a brand name. And also one I'm not familiar with, I think it's not so well known in my country. Tectyl and Hagmans are more common in Norway, and they probably have a similar product as the Waxyol.