When I'm painting high-gloss oil-based on woodwork (architraves, skirtingboards, windows), I have been following the tin's advice and sanding between all coats. I don't resent sanding after the undercoat/primer because it helps provide a smooth finish, but the final sand often takes the paint off sharp edges, even when I am careful, which can leave them a bit dark after the final coat. Is this final sand necessary?
It depends ;)
I don't tend to do a final sand for the very reason you mention. However there have been times when the finish hasn't been to my liking so I have sanded - but in these cases I use the finest grade paper and sand very lightly to just take off the drips and/or dust flecks that have settled on the wood.
So if the penultimate coat is good you might be OK without sanding, but sanding does provide something for the final coat to bind to if the previous coat was applied some time ago, so ultimately it will provide a more durable finish. If you are painting the second coat as soon as the first coat is dry then you don't need to bother.
IMO it's not worth it. Sanding has two primary goals. First of all, it compensates for the surface defects. In case of woodwork it also removes the wood fibers that get raised from the surface when applying paint and are fixed still in the paint layer and thus make the surface less even. Since this is not the first layer of paint you have likely compensated for all the defects and you've surely sanded away all raised wood fibers, so you will likely not get any real benefits from sanding.
From my experience - I've coated wood table with two layers of oil-based lacquer. Sanding made huge difference for the first layer, but after I applied the second layer the surface was as even as before the second layer. Definitely I wouldn't sand the second layer if I wanted to apply the third one.
I think there are three good reasons to not sand between the last and previous coats of oil based paint as long as the earlier coat has not dried very hard over an extended delay.
- You don't want any dust between the two coats;
- You don't want to remove paint thickness, after all the point is to put paint on, not take it off and pollute the air unnecessarily;
- With regard to sanding producing a key, that is partly necessary upon very hard surfaces, and those which may have some contamination like grease fragments or dirt.
I would just sand where the recent earlier finish is imperfect, i.e. dust, hair or other imperfections, then be sure to remove the dust with a microfibre cloth, better than a tack cloth which can leave a residue.