Yes there is a seal it is the link below. You have to buy 25 feet min. I might still have some but I would have to dig. The seal is for external frame to fuselage, not for window to frame.
http://www.cessnaparts.com/search.cfm
No, I did not remove all the rivets. As you can see in the photo, the bottom of photo shows the rivets still painted original. I was able to slide out the old window and slide in the new. I did not seal with anything until the window was installed and after new paint was applied. Then the Pro Seal was used around the exterior of the window where it meets the frame. The only way to seal your windows, all the windows. Especially if you are going to paint because then the paint can be applied to the very edge of the frame with no sealant mucking things up. Done right it looks like Gulfstream Jet quality. I also sealed around the fuselage frame to try to keep things dry and draft free inside. See photos, then ask questions.
The interior window requires the grey channel rubber gasket. I have resealed this E Window twice now and it still leaks a little when flying in precip. Pain in the A__. Still this E Window is not as difficult to change as the Door Window. :0