We've just sanded down the walls in the dining room. I got a bit
carried away leveling the edge of the vertical pipe boxing in the
corner, so parts of the batten and the edge of the ply sheet are
exposed. The ply face is still covered and the other edge of the ply
is in the corner, so no problems there.

The boxing had previously been painted with the same emulsion as the
rest of the room and blended in fine, so we'd like to do that again.

Obviously, I'd prefer to just emulsion without undercoat, but is it
necessary to use undercoat on the batten and ply edge first? Will
that create problems when we emulsion over it? Do I need to sand off
all of the old emulsion first?

Thanks.