We painted our antenna radomes with Aerodur, but it's a $200/litre fast curing two part paint that is quite dangerous to breathe. It wasn't chosen for its electromagnetic properties, just its toughness, but gave no trouble, two or three coats, up to 6 GHz.
I'd recommend any auto body paint, like a 2K two part paint. With the right primer and surface preparation it will stick well and last a long time in the sun. We also used these for less military applications, and when we needed a gloss white finish which we couldn't get in Aerodur.
We used a lot of black paint for antennas, but perhaps it's safer to avoid it, it is possible they contain (macroscopic) carbon which could affect the antenna.
You definitely don't want to load the antenna with any substantial thickness of plastic, like putting it in a new pipe. A layer of good paint is fine, any thickness, but a plastic pipe is not ok, might upset the tuning.
Finally, you don't want to seal it airtight - make sure you don't paint over the drain hole in the bottom of the original radome. A large volume needs to breathe and drain condensation, attempting to make it airtight will just cause it to suck in moisture from somewhere unexpected.