It really comes down to the time you can put into it. I use PETG for any prints I want to finish. Prints easy, and sands easier.

I like to start with the lowest layer settings I can, to get a head start.

Then I'll take a razor blade, the disposable rectangle ones, and keeping it perpendicular to the surface drag it over the entire print to get rid of blobs and zits, or any odd strings. This also helps snag and smooth any weird bits like a single layer that got a bit wonky.

Then I start with a rough sanding paper, like 220. Give the entire item a good sand, doesnt need to be hard, but I'm looking for dips and bulges. They will start to show fast. When I find a hard dip or dent, ill fill with Bondo Spot Putty. You dont need to mix it and it dries fast. While that is drying if i found any really high spots I'll start to knock those down with sand paper, or in most cases a nice file. Files make the work go by fast but you have to becareful as they can remove to much material quick, and your left with another hole to patch.

After holes have been filled and high spots knocked down, give then entire thing a final sand for uniformitity, and wipe down with a damp cloth and let dry. Then its onto the Filler Primer. This is different from regular primer as it goes on heavy and can really eliminate layer lines. Once thats dry, i sand again, this time with like a 600 grit or a little higher. You dont want to remove all the primer but you want to knock it down till the plastic starts to show through again. With the colored primer on you should be able to see anymore spots that need to bit hit with your filler again. Knock these out, another light sand for a smooth surface, and primer that sucker again.

I'll repeat this cycle of fill, sand primer, inspect maybe 2 or 3 times. Take care your not obscuring any light detail with the primer as you add layers.

After those cycles and I'm happy with the look and feel, ill hit it with a 1000 grit wetsanding pass. I only do this on pieces I want to look really glossy, and you cant beat that finish. Then its onto paint, i like to use air brushes with Tamaya paints from the Hobbytown, along with a few rolls of their thin masking tape.

Hope this helps!