How do you remove concrete splatter off a metal door?

You can try a diluted muratic solution 50/50 it has to be rinsed well,use gloves ,cover the floor with plastic,eye protection, adequate ventilation. Brush it on sparingly it should remove thin deposits instantly,rinse with water on a rag several times if a hose cant be used. Best way remove door and do it outside. Water based paint may come off its a hard problem.