I don’t think you do need to prime or use protective coat. I use Rustoleum on yard art things - I just clean dirt off the piece & set up my spray booth, don goggles & face mask & spray away. I do recoat things after a couple of years if they show too much wear from weather. Your tea cart will be protected so should last forever. If you want a very smooth finish & there’s lots of bumpy rusty stuff on it then yes, I’d presand but not after coat. Do be sure to use eye & mouth protection - you’ll be amazed at the particles you see floating airborne even on a still calm day.

I prefer the softness of satin finish over gloss. Will provide equal protection.

If the current finish is still adhering, I would just lightly sand, clean, and paint. If the current surface is rusty, I would clean the rust off, prime, and then paint. Many spray paints are now available with a built-in primer and paint.