Rat cage cleaning question?

We have rats. We have two cages that are plastic bins withe a wire top and wood planks and ladders so its three stories. We love these cages but we have one problem. The rats pee on the planks of wood. Its disgusting. I read that rats pee in one corner or area but these guys (two males in one cage and two females in the othee) pee everywhere. The girls are a little cleaner but still pee everywhere. The wood is unfinished. At the moment we have removed them for cleaning in hopes that rats will sort of "potty train" themselves. Any tips on how to train them, finish the wood with a pet-approved sealant, or keep them clean at least?