During heavy rain, water seeps through a hairline crack in my basement floor. Can I use Sikaflex Crack Flex Sealant to repair it?

Yes, you can use Sikaflex Crack Flex Sealant for your application. Please note that if the hydrostatic pressure is great, the sealant may fail. This would be the result with any sealant. The minimum depth and width requirement of the crack must be at least 1/4" x 1/4". If the crack does not meet this requirement, you can use a chisel and hammer to widen it. Properly prepare the crack before applying the sealant.