Nearly every old house has horrible plastic masonry paint covering the brickwork. In the past, removal has been almost impossible. These days, high temperature pressure washers allow plastic paint removal - and other methods can then be used to remove distempers and lime based paints to reveal clean historic brickwork. These methods do not damage the face of the bricks.
It is vital to ensure that any cleaning does not affect the glaze on the surface of bricks - if this is damaged, moisture can get into the brick and cause spalling. This is why grinders or mechanical means, including sandblasting, are not a good idea.