The value in the chairs as a whole is the design and the Finn Juhl / Vodder logo. Also the difficulty in locating a set of six in any condition. If I planned to keep the chairs I would replace the leather and clean the wood and use them happily in my home. If my plan was to sell the chairs, then I most certainly would not do anything, allowing the next owner to do what he/she choose. Cleaning and reupholstering will not lose one dime of value if done appropritely with the right material. Most buyers just want the chairs to use in their dinning area as they are just gor-geeee-ous.
Whatever you decide to do don't sand and you'll be ok. Sanding will change the original design in one way or another.
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