I'm a decorator and have agreed to fill a small chase for a customer in their new kitchen where they've had a socket put in (the chase is around 1.5 linear metres). I think it's an external wall, though am not sure as although it's a terraced row, the houses are staggered. I was thinking I would use a sand and cement render initially instead of bonding plaster (due to it being an external wall) and was wondering if I need to add a waterproofer/plastercizer? If so, can anyone recommend a small, inexpensive product I can use? Since it's such a small job, I don't want to have lots of big bags of plaster/render/additive left over. As a result, after rendering I was thinking of finishing it with Easi-fill, since you can buy it in small bags and I can always use it on another job. I was wondering what people's thoughts are about this - any advice much appreciated!