Additions of a strontium modifier will improve feeding during solidification, thus minimizing shrinkage porosity. If you don't get the results you need with the Tibor grain refiner alone, you may want to consider a trial with both Tibor and Strontium. If so, I would suggest a strontium...
I would agree with the previous post, you can improve the quality (leaker reduction) by the additions of a grain refiner (Al-Ti-B) and a modifier (Sr-Al). I suggest contacting AFS for information on suppliers.
In addition to the above, there may be other issues related to melt cleanliness...
