[header] How to waterproof fabric [title] Plan to waterproof your fabric on a dry, windless day. [step] 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. 
[title] Clean the fabric if it is dirty. [step] If the fabric cannot be washed, and it is only dusty or lightly soiled fabrics then clean it a vacuum or a brush.