Hey! So the best water- based paint sealer is Final Seal by Ben Nye- http://fxcosplay.com/products/ben-nye-final-seal
I would recommend spraying a layer of this on the skin and let it dry before you apply the paint. This is going to create a barrier between the natural oils of your skin and the paint. Then apply the paint, let it dry, and add another layer of final seal on top. This is going to lock in your paint and make it last all day until you use soap and water to wash it off!
Here is a great video on how to apply paint to a large area to make it look smooth and even and incorporates the Final Seal method I was referring to- https://www.youtube.com/watch?v=p98_WrOkUdc
Hope this helps!