Maybe I am just lucky, but I don't have this problem. My original C&C spar does not seem to leak. I keep my boat on a mooring, and perhaps having the wind always come over the bow helps.
I followed Don Casey's excellent advice and used a section of a tractor tire tube for a mast boot. (It has to go on before you step the mast.) This is clamped to the mast and to the mast collar with a stainless hose clamp, and the luff groove is filled with goop at the the top of the boot.
I haven't done it yet but I am making a canvas "over boot" that will hide the inner tube section from view and block UV that might degrade the rubber.
The whole deal is shown at page 24 of Sailboat Hull and Deck Repair by Don Casey.