[island911]

Yeah, you just need to test the pressures.

Typically external pressures won't be a problem as that compresses the layers.

For internal pressures it all comes down to how well the layers adhere. Obviously more dense infill will give more strength, but you will need to test.

Also, perhaps start printing with PET clear. Settings can be a bit tricky but it's the most common food-grade plastic, and you can see inside ... see what's happening.

Long ago I printed a little thin-walled calibration box (30x30mm 1.25mm wall) out of clear ABS - to check the scale factor. Anyway, because your post, I grabbed that box and just tested it for water tightness. Yep, no layer leaks. No leaks at all.

I wouldn't try to coat anything. Maybe 3D print a pattern for a urethane casting (if small volume) or go to injection molding if you need hundreds. Side note, injection molding may be weaker around the cartridge than the print. (hoop stress and injection flow knit-lines...)

Those half-cartridges are neat. I've design food-grade equipment components with those. I'm trying to think if I've ever prototyped with a 3D print on those. ...don't think so. pressures too high on that stuff.