This can be achieved by priming the board with Thistle Bond-it; although this negates the reason for using MR grade board, as skim plaster does not provide resistance to moisture.
However there may be situations where you wish to use MR grade board without the moisture resistance performance when in use. For example, where MR grade board options are used in shell and core construction to provide temporary resistance to high moisture conditions, they can be skimmed at a later date after the building envelope has been made weather-tight.
Pre-treatment with Thistle Bond-it is required when using Thistle BoardFinish or Thistle MultiFinish. Pre-treatment is not necessary if using ThistlePro DuraFinish. Please refer to White Book Chapter 08 Section 02 Page 06 for more details.