Hello I use this cheap convination of products to seal the seams of my poncho and tarp.
It works great with nylon based fabrics, I suppose that should work to with cotton fabric.
Mix:
50% Silicone (yes the product that you use to fix cristal windows.
50%paint thicker (yes the product you use to light paints)
Mix both till silicone is completely disolved them you can apply it with a piece of sponge over the seam, wait 24H and yo´ll get a seam seal.
Try before full aplicaton to check no problem with fabric.
Enjoy
It works great with nylon based fabrics, I suppose that should work to with cotton fabric.
Mix:
50% Silicone (yes the product that you use to fix cristal windows.
50%paint thicker (yes the product you use to light paints)
Mix both till silicone is completely disolved them you can apply it with a piece of sponge over the seam, wait 24H and yo´ll get a seam seal.
Try before full aplicaton to check no problem with fabric.
Enjoy