You can buy waterproofing sprays in camping or outdoor stores. Waterproofing sprays for tents also work well with hammocks. Aside from repelling water, it also protects your tent or hammock from developing mildews. The one made by 3M has an effectivity that can last for a year. There is also a waterproofing agent that you mix in the washing machine during the rinse cycle together with the fabric of your hammock. Make sure that the hammock is clean before you add the waterproofing agent.