Many waterborne urethanes bond like nobody's business so some dirt down it a crevice isn't going to hamper the finish's ability to bond. But to absolutely ensure no problems, all one needs to do is clean the floor thoroughly (vacuuming, washing, maybe even with mineral spirits) then after the floor is completely dry and as clean as can be gotten, apply a coat of Zinsser Seal Coat. This is a wax free shellac. Shellac is know for is fantastic ability to bond to pretty much anything and anything will bond to it. No buffing is required. After adequate dry time, apply the finish of your choice. You can continue with this process indefinitely. And those lights spots caused by wear, simply re-stain and touch up those areas prior to the recoat. Problem solved!