Water seeping in through cracks RadonSeal does not fill in cracks or openings. But apply RadonSeal first - it will penetrate inside the concrete, and help bond with any patching material. Since hairline cracks often do not penetrates all the way through, this may stop the seepage. Afterwards, handle the cracks if still necessary. You can use very thin Portland cement to fill in the cracks, keeping it wet for a couple of days.
Hardware stores sell special compounds to plug up even leaking wide cracks.
No, RadonSeal stops further efflorescence. RadonSeal can be applied over existing efflorescence to stop its growth. Often, it will purge efflorescence from inside the concrete. Then, you can remove the efflorescence if necessary. Thanks to the Internet, you can buy RadonSeal directly from the manufacturer or in several areas, from professional applicators and distributors.