Can I apply Thistle MultiFinish to sand & cement undercoats?
Thistle MultiFinish would be suitable although suction may need to be controlled (e.g. with Thistle GypPrime). The best finish to apply depends on suction level of the background material. Suction and background recommendations for individual Thistle plaster products can be found in our plaster selectors: Thistle Essential Selector Guide, ThistlePro Selector Guide.
Cement based undercoats shrink on drying, usually with some cracking, which can appear several days or even weeks after application. If a Thistle finish plaster is applied before the shrinkage is complete there is an increased risk of delamination or cracking of the finish, particularly if the undercoat was not thoroughly keyed. The key provided to cement-based undercoats needs to be much deeper and the drying time allowance much longer than for gypsum-based undercoats.