Have you ever tried to paint over Blu Tack and the stain still shows through?
Blu Tack contains a slight oil base which helps with the product release from a wall or other surfaces, quite ingenious until it comes to redecorating or removing the oily residue from the wall. Sadly the oil in Blu Tack penetrates into the surface of many paint substrates, so even if Sugar Soap, or various other degreasing agents are used the oil is still within the paint layer, and it will slowly make its way to the surface or through freshly painted surfaces and show as a shadow mark again.
The way the team here at Chicoco Decorating & Property Care tackle this problem is to use a stain block, namely Ronseal Stain Block. This product has a white base colour and paints over really well. First give the area of the blue tack stain a very gentle sand with a 200 grit sand paper, just lightly as there is no need to take the paint off, next brush or roll a the affected area with a coat of Ronseal Stain Block and allow to dry completely. Finally another light sand and you're ready to apply your top coat without the Blu Tack stain or marks reappearing through your new lovely paint.