Sealants require a gap of about 6mm to work well, smaller gaps will just pull away over time as there is not enough material to stretch and contract.
If they are cracks, then covering the outside with waterproof ply wood or ship lap, should do the trick.
Pro-seal34 is a good waterproof sealant that will work on a lot of surface.However as said above it will be inefficient over time on very small cracks.
Hope it helps and let us know if you found a better solution.
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