Repairing Cob
Cob is a remarkably durable material provided it is treated correctly. In practice, this means keeping it dry. Unfortunately, as ordinary portland cement has taken over from lime in the building trade, largely during the second half of the 20th century, builders and home owners have often made the mistake of thinking cement (being very strong and waterproof) will protect the cob well. However, the reverse is true. Although cement will indeed keep off driving rain well, it is very brittle and often cracks due to movement in the much more flexible substrate. These cracks then let in water which is trapped by the impermeable render. Thus, rather than evaporating away when the rain stops, this water builds up in each subsequent storm, usually sinking to the bottom of the wall where in extreme cases it can eventually lead to total failure of the cob, i.e. collapse. This in a wall that might have sat quite happily for over four hundred years prior to the addition of a cement render.
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.