Tools with wooden handles get abused. Rakes, shovels and garden tools are always exposed to the weather. Hammers, saws and woodworking hand tools get beat up. Wooden tool handles are rarely coated with any protective lacquer, and even if they are, it wears off and they dry out and crack. Penetrating oil is the best way to treat wooden handles. It penetrates into the pores of the wood and hardens, protecting the tool from the inside out.
Sand the handle by hand with 100-grit sandpaper. Form it into a cylinder and slide it along the length of the rake or shovel handle to remove all gray weathered wood or loose fibers. Swirl it around hammers and small tool handles, making them smooth.
Dip a cloth into the linseed oil and liberally apply it to the wooden handle until the handle will not absorb any more oil. Wait 15 minutes and wipe the oil off. Wait 24 hours.
Apply another coat of oil to the handle. Wipe off the excess oil but leave the handle looking wet.
Apply another coat of linseed oil every six months.
If you have splinters in the handle, break them off first and then sand the handle. If your handle is cracked or split. Inject glue into the crack. Put a hand clamp on it and wait 24 hours before you sand the handle. Boiled linseed oil dries faster, but adds no extra protection, use it if you desire.
Although linseed oil is non-toxic, avoid breathing the fumes.