Can I just use undercoat on bare wood?

Yes, you can just use undercoat on bare wood, and in fact you should use undercoat or primer on bare wood before topcoat for best results, but you should not use undercoat alone as final finish. Bare wood is porous and needs sealing, so applying undercoat or wood primer first is correct practice. Undercoat will seal wood grain, block tannins and knots from bleeding through, provide adhesion for topcoat, and ensure even color. For bare wood, best undercoat is oil based or shellac based wood primer that blocks tannins, such as Zinsser Cover Stain or B-I-N, or acrylic wood primer for low odor interior. One coat of undercoat is usually enough, with spot second coat on knots. After undercoat, you must apply topcoat such as paint, as undercoat alone is not durable, not washable, and not UV resistant, and will look flat and will absorb moisture. So you can just use undercoat as first coat on bare wood, meaning you don't need other prep beyond cleaning and sanding, but you must follow with at least one to two topcoats for protection and appearance. So yes, use undercoat directly on bare wood as base, then topcoat, do not leave undercoat as final finish.