Bentomix oil based sand will bake and become granular in the area of contact with the metal and as such the baked portion should be removed as it will not bond together and being granular defeat the nature and whole point of the fine sand. Our suggestion is that you having put parting powder on the surface of your pattern you cover it with a layer of 5-10mm Bentomix and then back fill the moulding box with the Mansfield sand. It is advisable that you on breaking out the casting, remove the portion of baked sand thus enabling you to reuse the Mansfield sand.
When preparing the Mansfield sand by adding water it is advisable that you retain a portion of the dry sand to enable you, if you have over watered, to obtain the necessary consistency to allow the sand to bind together. As a rule of thumb if you take a fist full of sand and squeeze, the sand should bond without excess water being expelled. 
The best result can be achieved by surrounding the pattern with a layer of Bentomix and then back filling the moulding box with Mansfield.