I was just wonderin meself the other day what the best way to treat em was, and thought what was needed was a substance which sets like rock and is impermneable to air, water and detergent, and stays on forever. The only answer i can think of in this respect is Dried-On-Weetabix, so i've ordered a bulk delivery from Asda!
Seriously though, it needs to be something that is relatively flexible and bars air and moisture, there are high zinc content paints which some say are quite good, Waxoyl needs to be done regularly as it flakes off, the Cleantec chap generally gets bad reviews for missing lots of important bits, but the most important thing i recklon is to get the mud off it as quickly as possible as the mud holds the moisture on it - using LOADS of clean water.
Good luck, it's an uphill struggle!
All the best
OD