My timber window has been painted by a decorator. It was stained and varnished previously. The decorator used shellac to seal it prior to painting white. The paint has flaked off the filler around all of the windows leaving an exposed brown filler. I think this could be caulk - it is flexible. I have now cleaned off all of the flaking paint, but do not know what to use to seal it to enable me to repaint. Can you please help?