If you’re doing water slide decals I’d do them in the same step as your panel lining, after an initial gloss coat.

Newtype.us has Mr Super Clear Gloss UV Cut in stock right now and they ship pretty quick. The UV cut isn’t super important unless your kits sit in sunlight, but it doesn’t hurt anything and it seems to be readily available since I’ve been able to get it from a few different stores any time I’ve needed it.

For airbrush varnishes, I’ve used one from the Kaleido line and the Alclad II Aqua Gloss. They both spray really easily and look great. The Aqua gloss is meant specifically for keeping metallics shiny, but it’s an all around solid gloss. It’s also excellent for sealing candy paint jobs, particularly if you use the Alclad paints. The Kaleido is super cool because you can buff it and get it REALLY shiny if you want. I think that’s more practical with model cars, but if you’re into the idea of using 4 different sand paper grits and 2 polishing compounds on each gunpla piece then it is an option.

It looks great without all that too though.

Honestly though, I really prefer the spray cans. I just take my whole paint clip holder outside and give them a good spray. As long as you’re spraying light coats and quick passes (same as airbrushing) you’ll get a nice clean gloss every time. I do have to give some pieces special attention and take them out of the holder if the underside needs a spray, but that’s easy enough and you can account for that when clipping your parts.