Recently, I bought my first Sackville Rollo handlebar bag. Well before I received it, I did some homework about how I could waterproof the canvas more, since I live in a very humid and wet climate. I often ride in the rain here in the PNW. As far as the protection of the leather on the bag, I use Obenauf's HP Leather Conditioner. It works very well. I use it on my leather shoes.
What I came up with is a product called 303 Marine Fabric Guard, which repels moisture and stains. It is safe for all fabrics. As to if it fights off mold and mildew, it impedes and hinders the formatoin of mold and mildew. It does this by restoring the fabric’s water and stain repellency, which keeps the material dry and prevents dirt from soaking in—creating an environment where mold is far less likely to grow.
I do store my bicycle and my handlebar bag inside my home when I am not riding.
Kim Hetzel.