I used to wear a thin inner PU coating jacket as my rain sell, but I found it is not waterproof enough under heavy and long time rain. Thus, I decided to apply silicone coating on the outside of the jacket by myself. I imitated the seam sealing method posted by Jay on this site using thinned silicone.
Ordinary clear silicone sealant but not silnet was used to mix with paint thinner, and I found it does work as good as silnet as Rojer mentioned. It is a little difficult to decide and keep the thickness of the solution since I found it become thicker and thicker during the painting work. I believe thinner solution is easier to go into the
seams but is not that waterproof and durable. In addition, It's so hard to paint evenly that some parts of finished fabric face is just like ordinary silnylon with slippery texture but othe parts are just similar to the dry silicone sealant which touches a little sticky (the sticky feeling is not really sticky).
This jacket is 5oz. before applying the coating and 5.6oz. after. Forgot to take pictures during the coating process, but have one for the finished jacket. It looks just like silnylon jacket with oil luster, but I think it is more waterproof because I've painted several times, and it ended up with thicker coating.
I think this method can be treated on outter fabric of any backpacks to enhance the waterproofness. For example, the frequently used dyneema or dyneema X gridstop fabrics are strong but not so waterproof. I guess applying silicone coating will make them more versatile.