I want to use white gloss to paint some white doors inside the house.

The doors were painted with white about 15 years old and are getting
discoloured.

I know preparation is important but in this case I have to compromise!

I'll use white undercoat and one topcoat of gloss. The undercoat is to
even out the surface and prevent discoloured patches showing through.

QUESTION: If I can use either sugar soap or fine sandpaper to prepare
the surface (plus filler for holes) then is it better to use fine
sandpaper than sugar soap?