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How to waterproof fabric
Plan to waterproof your fabric on a dry, windless day.
You will be working with spray-on sealers, which are sensitive to humidity. Also, if you are working outside and it is windy, you might get some dust embedded in your fabric.
Clean the fabric if it is dirty. If the fabric cannot be washed, and it is only dusty or lightly soiled fabrics then clean it a vacuum or a brush.