For over 20 years I have been making and installing counters from time to time and have used most of the potting equipment available. No tensioner is strong enough to compensate for poor corners or gaps caused by poor edge bonding. What they do is close a good seam, and the FastCap version does this very well. Get an affordable FastCap recess routing template. A good fit for your counter is marking and creating templates, not what type of seam sealer you use.