With regard to sweat stains, the same visual checks as mentioned with dirt should be implemented, and again, follow the cleaning instructions on the product's label. Mold and Mildew Mold and mildew are perhaps the most common ravagers of bivy sacks and tents, and their effects are easily avoidable. Of course, you can't always break camp when things are dry, but when you return home, make sure everything is dry before you store it away, especially a bivy sack. Mildew has the capability, over time, to weaken nylon such that when you remove it from its bag it will be full of holes. The initial stages of this end-result catastrophe include a rank smell.
Remedies are available.
Perhaps one of the best is a mixture of 1 part lemon juice concentrate and 1 part lukewarm water, with the concoction subsequently sprayed from an atomizer, e.g. a spray bottle used for plants or ironing clothes. Spray the affected area with a moderate dose of the lemon water mixture, let stand for 5 minutes, sponge off, and rinse well with lukewarm water. Then, for best results, sponge the area with a cleaning product that coincides with the item's washing instructions, and rinse well.