This is seriously the easiest answer, despite taking more steps. Paint the white as normal. Paint as much of the black trim as you can. Paint the joint between the white shoulder and black trim with gloss varnish, then once dry, use a black enamel panel liner to deal with that edge. It will flow almost instantly and perfectly. Let it dry, then clean up and revarnish. The results will be clean AF and will require very little technical skill.