There are several thin cracks on the refractory panels inside my pre-fab fireplace, and the chimney sweep guy recommended fireplace mortar. I went on Rutland company's website, and their fireplace mortar is for patching fireplace fireboxes and firebricks, for cracks 1/8" or less, and once cured, will be as strong as the firebrick itself. Has anyone had success with this product? :confused:
Thank you.

Yes but only moderate. Your panel was fairly strong from the factory and it still cracked. A patched panel will not be as strong as new so it may crack again, just repatch. The job is easy and the pact is cheap. You could raise a stink with the manufacturer, I have had them send me new panels free of charge.