What is a good way to refurbish the cabinet floor under the kitchen sink?

Is the existing cab floor water stained,rotten or warped?

If water stained and made of wood; sand, stain color of cabinet and seal with clear poly.

If rotten or warped, replace with 1/4'' plywood and finish as stated above. U may have to put plywood in 2 pcs. Be sure to support seam.

Good luck

Chuck

For these type repairs I usually use 1/4" Luan. It is cheap and really only 3/16 thick. A couple coats of polyurethane seals and makes a nice finish. As Chuck said if you have two doors and a center style, you will probably have to make it in two pieces. Check the back side of the center style. If it has two screws in pocket holes holding it in place, it can probably be remove and reinstalled.
Cut and fit material, apply a couple coats of finish and install. I usually just glue it in place on top of the old cabinet bottom with construction adhesive around the perimeter and a bead down the center..

If the floor is still good but looks bad, I usually get a small piece of scrap vinyl flooring and dry lay or edge glue it over the old floor.