Which product do I use to treat garage walls that " sweat "in the winter
You don't need waterproofing, you need insulation.
Unless your garage is underground, &quot;sweating&quot; is actually condensation, and it's due to the walls being cooler than the air, and below the dew point.  One way to prevent it is to dry the air, but dehumidifiers consume energy (= cost money) to run, and chemical air driers cost even more over any significant time period.
The better way to prevent this problem is to ensure that the interior wall surface doesn't get below the dew point, by keeping it close to the inside air temperature -- and the way to do this is to insulate.  Reduce the rate at which heat is lost through the walls, and the warm, moist air (presumably infiltrating from the heated house) won't have a place to deposit moisture.
This isn't a super-easy DIY job, unless you're the sort who could build the garage in the first place -- but if it can be done from inside (by removing drywall) instead of from outside (removing siding, vapor barrier, etc.) it's not terribly difficult, especially if you don't mind seeing joints and nails or screws in the inside dry wall.
The inexpensive product (in my experience, which is pretty old, so check behind me on this) is &quot;rock wool&quot; or fiberglass insulation batting, and a recommended R value for an unheated garage is fairly low -- R-14, perhaps, to R-18 -- but if the garage is connected to the central heating, it should have been insulated when built, and the recommended values today are something like R-24 to R-32 (again, check me, I'm neither a contractor nor a young fellow and these values tend to rise with time as energy costs and environmental concerns rise).