Build Something Beautiful

The answer is to stick to either no render at all or a lime putty-based render, as has been used literally for thousands of years. The same applies internally; either lime or (even better) earth plasters work best. Most of my work repairing cob buildings over the last thirty years has been to strip off cement render and carry out necessary repairs, sometimes replacing existing repairs in a more compatible medium, namely cob blocks and an earth/lime mortar. The render or plaster can then be replaced with lime or earth mixes, before being decorated in lime wash.