At what point does a NEW, semi-gloss, latex paint become an OLD paint finish?  Another words, when painting over old paint, it’s recommended to cut the old sheen so the new paint will stick.  Most manufacturers recommend waiting on a second coat for 4 hours.  If it’s been 12-20 hours since the first coat, is it OK to paint over the old semi-gloss, or do I need to sand lightly?