Can this be used for an undermount sink

Thank you for the question. It is possible the Loctite Power Grab Ultimate White might work but it is not recommended for safety reasons. A construction adhesive should never be the sole source of support for under mount sink installs. The successful use will depend upon several variables such as the composition and condition of the counter surface underneath. The composition of the sink itself, if this is a replacement sink what was used previously or if this is new installation. There are other factors to consider but in general construction adhesives should not be used as the sole means of support in under-mount sink applications. Under mount sink clips or other mechanical support should also be used for safety reasons.