Seam Sealing product are designed to "refresh" the waterproofness of a material not really to make it waterproof. It might help and certainly won't hurt but it won't make a non waterproof gear become waterproof. Seam Grip and Seam Sealer were really designed for polyurethane coated material like tents' fly. I won't even bother with water repellent treatment as I doubt it will help at all. If it does help it probably won't work for more than a few days. You may want to consider buying panniers cover, I know Arkel makes some for their panniers as they are not waterproof but they are pretty expensive (around $40 I think). Otherwise, even the cheapest of the Ortlieb panniers are very reliable and waterproof, and a pair is about $110. Not cheap but a good investment in my opinion. Good luck, if you end up trying the seam seal, please let us know how it worked out.