Strip and finish off the boards before planning. Not only will it dull the blades, it'll probably **** up the planer. The dust from the finishes is also deadly. Reclaiming lumber takes a lot of work but definitely worth it.The varnish on the wood can dull the blades so be prepared to buy new ones after the job is done. Also, be sure to have an outfeed table or a roller system to hold the weight of the wood. An infeed system would be a good idea too.