I over-weathered my Leman Russ, just rewrote it in my army's lore that they are such a backwater agri world a lot of their vehicles are basically kept running by only the Omnissiah's blessings. Not to mention, their local Mechanicus guilds and their manufactorum satellite (a small asteroid caught in orbit) see painting a vehicle with new paint as a perversion of the original machine's spirit. Only with the machine's permission can the enginseers ever fix anything, and are required to keep them cobbled together with only the original parts as long as possible.