I have several old solid wood doors. I want to refinish them and use new hinges and doorknobs. How is the best way to repair existing hinge cutouts and holes and door cutout and holes? Thank you. Jean
1/8th" Luaun plywood from the big box store and a good chisel should let you cut out very tight fitting patch pieces with some practice. That'll get it into paint shape. For a stain finish, you would have to try to match the grain and color, which would be a bit harder.
I have very good results filling lock bores and hinge mortises on doors, and latch holes in jambs, using 2 part wood filler. It's polyester resin filler similar to automobile body filler: bonds tightly, easily shaped with "cheese grater" planes and sandpaper, cures hard, will not shrink, can be drilled/screwed into/chiseled, etc.
Works great as long as you will be painting.