Nothing wrong with a horse sweating well when exerted and/or heated appropriately! Downside, however, is that this sweat does not exactly benefit the durability of a leather or synthetic harness, due to the (slight) acidity and minerals present. Fortunately, there is a practical solution to this by using underpads. So these offer even more benefits than comfort alone, which is why they are available for almost all harness components and in various materials and colours.
Saddle pads
So the idea that only a leather harness needs to be protected from the action of salts from sweat is incomplete. Plastic harnesses also last longer if protected by a chest pad and/or an withers pad.
