I put in 3/8th sheetrock over an old cracked ceiling; finished my
taping and spackling and put on 2 coats of primer (it was the glossy
kind, all that we had) and then put on one coat of ceiling paint (flat
white) before realizing that I should have done one of the seams a
little bit better. I guess I felt rushed, but anyway, I'm hoping to
rectify my error by trying to even out this seam, which is simply a
bit depressed than the surrounding boards, by using a 6 inch spackle
knife to apply another thin coat or two to do a better job.

Is there any specific way to approach this? I would think that once
the paint is on, there is something that needs to be done, but I'm not
sure. Hopefully, maybe just a little abrasion to roughen up the
painted surface so as to have the mud adhere? I wasn't planning to put
up additional tape for this last couple of passes with the larger
knife and some spackle.

Thanks for any help!
Paul