My #3 won't go through impacted or matted coats. If she's matted you will need to use a #10 (the blade most clippers come with). If she's really matted it's not worth the torture (to the dog) to brush/comb it out. Shave it off and start again with proper coat maintenance. The dog should also be bathed before clipping. Dirt in the coat will dull the blades fast and cause them to heat up which risks clipper burn.