Do you need undercoat when repainting?
Correspondingly, do you need to use undercoat when repainting wood?
the wood will need a decent coat of primer (or two) followed by a spirit undercoat. some of the better gloss paints are self-undercoating, however a primer is still necessary. and as it is bare wood i would undercoat as well, even if it is a self-undercoating gloss as it will help build up the opacity.
Similarly, do you have to remove old paint before repainting? If you are going to repaint your house and your new paint is different than the old paint, you should first scrap the whole old paint and then apply only new paint. If existing paints are whitewash and distemper, you should scrap the whole paint first.
Beside above, can I just paint over old paint?
Priming is best. If you have to do two coats of paint to cover the old color, why not use a good primer or primer/sealer and one coat of a good paint? Most people will say otherwise, but primer will stick to old paint a lot better than new paint will. Use one coat of primer and one coat of paint.
Can I paint over gloss without sanding?
If you want to update semi-gloss painted walls but don't want to sand and prime; or, if lead-based semi-gloss walls prevents you from sanding, it is possible to paint over semi-gloss walls with a satin-based paint without sanding or priming. If it's water-based, use a water-based satin paint.