I once had a Sinbad which has the same hull as the 22. I never had a problem like yours but I do know how its constructed. Cracks in the guide channels could be caused by a heavy collision with the bottom, maybe by the previous owner. If that is the cause your repair by reinforcing the outside of the channels is probably the best cure.
The other possibility is that the keel has been run up and down in the guides in the presence of sand or grit which has worn the insides of the channels. You could check this by taking the table top off and measuring the internal width of the channels right at the top where they get no wear, and comparing the dimensions lower down at the level where the cracks have appeared. If there is a difference indicating a significant thinning of the structure then I think the channels need to be built up on the inside and this would be a much bigger job.
Geoff
Geoff Harwood