Our kitchen cabinet doors have clear coat (unknown what it is) probably from the 60's. It has worn off in places and looks bad in general so I want to sand them down and put on fresh polyurethane. Do I need to take the doors down to bare wood before applying new polyurethane, or can I put the new on after a light sanding?
I'm a little concerned with the sanding because the doors are veneer.
Yes and no: it depends on the current condition of the cabinets and what final look you are aiming for. Also, when you say "veneer", I hope you mean a thin layer of hardwood, and not laminate which is a thin layer of plastic that looks like wood.
If your cabinets are in perfect, like-new condition except for the clear coat, then yes, you could benefit from stripping off the previous coat and applying a fresh coat. This will avoid all patchiness. It will also teach you why cabinetmakers charge more to do this than to sell you brand new cabinets. But if the cabinets are worth it, and you have the time, you could do this.
The alternative: you say you don't know the composition of the current coat, so you must do some tests. Find a hidden area of the cabinets with the same treatment, e.g. inside or doors, a shelf inside, the inside of the drawer face, etc. do some light sanding, e.g. 1000-grit sandpaper and apply regular poly on one area and water-based poly in another. If the regular poly sticks and looks good, you can experiment adding some oil-based tint to the poly or use some pre-tinted poly that they sell in stores. Play with it until you like the results, e.g. soaking the wood then wiping with a rag, or a light coat with a small brush, etc.
I've found that with most hardwood cabinets, a light sanding with 1000-grit sandpaper and a very light coat of water-based poly applied with a brush does miracles. So if these were my cabinets, my first test would be with water-based poly, and if it works, do that.
