It won't do what it says on the tin.Thompson water seal is for use on external brickwork to prevent the bricks taking up water and spalling in the frost.It will not be affective if used internally and plastered over (if I understand your post correctly).Dependant on the extent of the problem there are products available (RIW or Synthaproof for example) that can be applied to the wall and still provide a key for rendering / plastering.It will also depend on whether it is rising damp or penetrating through from the outside (high ground etc).If the problem is really minor then you can use a damp proofing agent in the plaster / render to help.All said, best to solve the reason for the damp than to simply try and hold it back.NB
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