For most people, flatting the paintwork is enough. There's a degree of risk, if you don't know the history of the paintwork. Going down to the steel is not what most would want to do, but it provides a degree of certainty.
Some DIY overpainting, from experience, suffers from lack of prepwork and keying up. It's the tedious side of boat painting, but important. When rubbing down paintwork, you'll generally go through a few layers, on the way to prepping and flatting. Each layer should feather, with a smooth transition between layers. If sections will not feather, the likely cause is bad prepwork and the layer is suspect - you should go below this layer.
In rubbing down, you need a good orbital sander, start with 80 grit, go on to 240, and 320 to 400 to finish.
Keep the brush strokes in the same direction throughout whole job, on each area. Apply with foam gloss rollers (4"and 7") and lay off with something like a 3" brush.
Get paint on, lay off, leave it - don't go back and fiddle. Keep surfaces clean. A vacuum is useful, attached to the sander and also with a soft brush to clean off dust from surfaces, prior to tak rag, and helps get dust out of crevices.
Don't put paint from your roller tray back into the tin that contains unused fresh paint. Any left-over paint should be tipped into a jar, or other, and can be used later. But, I think it's best to do your final coat with fresh paint. Contaminants, dust etc, are being picked up while you're painting. No point putting used paint back in with fresh.
Be sure to get rid of and clean the surfaces of silicone; around window frames, usually. It will be picked up on your painting equipment and get mixed in with your paint, spoiling the job. You will have to throw the paint away.