I think the answer to your question depends on your application. Is this Kitchen/Bath or is it something not subject to much wear/tear? GF has recommendations on when to use a topcoat and when you should use more than a satin poly. The GF milk paint is fine on its own. Their satin poly works well as a topcoat, adding some protection without changing the sheen much, like Wayne suggested. I don’t have experience with the other topcoats over GF milk paint.