Why does my gloss black finish on steel keep coming out orange-peely?

Cold garage is a big part of this. If the surface is cold, the paint doesn't level before it skins over, and that's your orange peel right there. Getting the metal itself up to room temp matters more than the air temp, because steel pulls the heat out of the wet film fast. I'd get a cheap infrared thermometer and check the panels, not the air. Also let the cans or gun and paint sit inside overnight so you're not spraying cold material. Dust nibs are a garage problem, period. Wet the floor before each session to settle dust, and strain your paint through a fine mesh filter even if it's straight from the tin. And back off the gun til you can see the surface go glossy-wet, then stop. Trying to build gloss with heavier coats just gives you sags on top of the texture. So yes, the answer is wet sand and polish, that's how basically every mirror-black finish is done, sprayed by pros or not. Nobody lays it down dead flat and walks away. You flat it back deliberately and then polish what's left. On furniture I'd sand through the grits from maybe 800 up to 2000 wet, using water with a drop of dish soap, and I'd guide sanding blocks or a foam pad rather than bare paper so you don't dig through edges. You'll know you're at each grit's done point when the scratches from the previous grit are gone in raking light. Then a polishing compound on a soft pad, working down to a finer one, and finish by hand with a clean microfiber.

And yes, black is brutal, it's basically a dark mirror, so every scratch you made becomes a highlight line. The scratches you're seeing now are probaly from the prep before the topcoats, wich is why you seal with a lighter grey or even a tinted primer and flat that back properly before you ever touch the black. Fix the prep and booth conditions now, accept that the last couple coats get sanded and polished regardless, and you'll get there without stripping.

Before you start flattening anything, give it a proper cure first. I've jumped the gun on this more than once and the paper gums up, the finish shrinks back into the scratches a week later, and you're doing it all again. We used to let black lacquer sit a good week before color sanding it, longer in winter. Modern stuff cures faster but still check the can, not just how it feels. And don't chase the polish too early either, work the grits until the last one's scratches are uniform, then the compound has something even to bite into. Half my dud finishes came from impatience, not bad paint.

One thing that saved me on a black table I did a couple years back was making proper spray-out panels on scrap steel and running them through the exact same process before touching the actual pieces, because otherwise you're tuning your gun on the graduation work itself, which is a horrible feeling. The other thing I'd add is lighting, since a single bare bulb held low and raked across the surface shows every high and low spot that overhead fluorescents will happily hide from you, and I only learned that after polishing around a dip twice.

Something I learned the hard way on a coffee table frame a while back: leave yourself enough film thickness to cut into. I put five coats of clear on because I knew I'd be sanding away most of the last two while flattening. If you've only got two or three coats down, you'll end up sanding thru to the colour and repainting anyway, which is exactly the thing you're trying to avoid. Check how many you've got on the actual pieces before you start cutting.

Update: took the spray-out panel advice and it changed everything, wish I'd done that from day one. Also started warming the steel before spraying like the first reply said and the orange peel is way better alredy. One thing though, the reply about letting it cure a full week before flattening, does that count from the last clear coat only, or from when I started the whole topcoat job?