Spray them. I have used https://www.halfords.com/motoring/paints-body-repair/car-spray-paints/halfords-clear-lacquer-500ml on 3d PET-G printed parts, not sure about nylon.UPDATE
Had a look at the blue 3D printed covers this morning and mine seem to be porous. Turned them over and ran some water into them and had bubbles of water permeating through the top. Wonder if this is the 3D print process as to injection moulded process.
Covered the top in gorilla tape and no water permeating. Replaced and will check again after some rain.
Grease is grease as far as I'm concerned, I'll just ask at the garage for a blob when I'm next there (service next month) as I'm not spending money getting a whole tube just for a blob!What type of grease are suitable for this location?
Quite. Which is why my first emergency fix a few years ago was a small upturned tuna tin that sat wholly outside the circular outer rim. Later I changed it for a cut-down yoghurt pot that wedged over that outer rim in your picture to hold it firmly in place. With the entire area covered there was no way that water dripping through that ridiculously placed drain hole could get into the strut top. Just covering the inner area still lets water settle around it and possibly splash underneath the cap as the car bounces around.I have noticed that some of these covers sit within the strut opening, others sit outside the formed opening.
I've found these tougher than the larger circular ones. No problems with leakage but following posts here have given them a coat of waterproofing stuff, just in case.