Does an old photo frame need to be sanded before painting?
You should give the old paint a light sand to provide a “key” so the new paint will cover well. a fine grade sandpaper is all that is needed.
A light sand is just enough to see gentle scratch marks in the surface and not heavy enough to go through into the layer beneath.
If you don’t then when you put the new paint on it may not cover easily or when it dries it can chip or peel off.