Thanks all. My chemist friend also cautioned against possible oily residue that would trap dirt. Petroleum products appear in a lot of waterproofing media so it may be OK for stitches but I intend to find out.
I have some old sunbrella covers from having had the boat reupholstered a couple months ago. They are a bit worn and have lost water beading ability so I thought I may take one and apply this thing and see how it works. Could be a cheap remedy, or could be a mess.
Fortunately, it rains a lot during the monsoon, so once I get the stuff mixed and applied we should find out quickly how well (or if) it works