I have a jug of twin/knot treatment that we use in commercial fishing gear. The active ingredient is "acrylic polymer" in a water based suspension. Depending on how dilute you use it, it can help preserve, keep clean and stiffen the netting. I've been playing around with it pretty much full strength (almost like PVA glue) as a way to make "stiff" braided slings.
A readily available substitute would be the glazing fluid that the faux-sponge painting types use. Should be cheap at the home center. PVA (white) glue might work as well, though mabe it would soften up with humidity/sweat? How about stockholm tar or the like (the burning pitch-ot guys could turn their attention to something less dangerous). Linseed oil or the like might work as well, though I seem to remember something about the acidity eating away at cordage.
Whatever you try, be sure to let us know! Maybe we need a comparison of different available treatments, initially as well as after a few months of hard use.
Matthias