Some years ago I had a backpacking tent with a leaky floor. I used a garden sprayer and Thompson's Water Seal. It worked but it added a thick layer to the floor and the smell was so bad that it had to sit out in the sun for a couple weeks before it went away.
Home Depot has some stuff called Olympic Waterguard that I recently used on a sun canopy. It had been previously coated and the Waterguard left a film on the old coating. It didn't soak in. I don't know what it would do on plain fabric. I haven't had it out in the rain since then so I don't know how effective it is.
