For the most part minor pet stains will lessen or get lighter with sanding. Repeated stains (the darker the deeper is the rule) will not sand out. Several consideration must be given. 1 - Will the floors be refinished ? If so, a light or medium colored stain can be use to help "cover or lessen" the stains. Sometimes a "painted" design on the floor will cover the stains, or the use of area carpets over those stained areas. 2 - Removal and repair will give BEST results, if time, and budget allowances are made.