The secret many working professionals use to simulate sweat on skin for long photo shoots is a mixture of water and baby oil, sprayed on the skin.
However this method could cause sunburn in hot weather
Other methods include
1. Applying a layer of suntan oil with some kind of a SPF factor, thus protecting the model, followed by a spray of water which should bead up nicely over the oil.
2. Glycerin. This works even better than baby oil, but difficult to find- best found at a chemist or local drug store.
3 canola/ peanut oil is another good option.
Don’t forget to include cleaning materials (rags, towels, ‘baby wipes’, etc..) 😉 And make sure your model has applied sunscreen *first*.