Seam Sealer

Marlen Textiles has tested a few commercially available seam sealers on the market and has found the best performing is Seam Grip WP by Gear Aid. 

https://www.gearaid.com/products/seam-grip-sealant-waterproof

While Seam Grip WP did perform the best, this is not an endorsement of the product nor a guarantee of its performance in your application or it's long term durability.