All Schnauzers have double coats: a wiry, dense outer coat over an equally dense (but softer) undercoat. Giant Schnauzer grooming is a moderately time-consuming process, because these dogs will need brushing 3-4 times per week, baths every 6-8 weeks, and the coat will need trimming every three months or so. Many breed owners (particularly those who show their dogs) strip the coats 2-3 times per year.
Brush your Giant Schnauzer in typical line-brushing fashion, making sure to wet the coat beforehand with mist from a spray bottle. A slicker brush is best for these dogs, and having a comb for working through stubborn tangles and mats is a good idea as well. If you plan to bathe the dog, give it a thorough brushing first, then use either a bathtub or an outdoor kiddie pool and a garden hose. Using only canine shampoo (as human shampoo can irritate the dog's skin), lather well, starting on the back and working downward; rinse thoroughly, towel-dry, then give the coat another quick brush-through to make it look neat and clean. (NOTE: in between baths, make sure to pay attention to your Schnauzer's facial hair, which can get pretty dirty when the dog eats. Clean the face with a damp cloth if it gets soiled.)
Giant Schnauzers need haircuts about every three months--but doing so brings up several issues. Regular clipping will change the texture of the outer coat over time, making it softer like the undercoat (which isn't necessarily a bad thing!); if your Schnauzer's outer coat is salt-and-pepper in color, constant clipping will turn its color either solid silver or solid black. Many owners also strip the coats, whereby the dead hairs are plucked by hand or removed with a stripping knife, several times per year. In addition, Schnauzer owners frequently trim their dogs' facial hair about once a month to keep it from getting dirty when the dog eats.
With all the above-named haircut matters, taking your Schnauzer to a professional groomer at least once is highly recommended. The groomer can advise you on dealing with these issues, and provide tips on how to groom a Giant Schnauzer in general.