for 7 years I use a vinyl type " stepped size" collar available Westmarine etc. Attached with long SS strap over silicone. No leaks ever.
Make shure your leak does not come from below the collar through a drainage hole etc you have not noticed.
I recently had to have my mast unstepped, etc.. When I talked to the rigger about Spartite, he said it wasn''t the right product, only to use it if you had a very irregular mast section. He thought that it was the best marketed product around, but not as good as what people thought. He will be putting some silicone and tape over the mastboot, and suggested to replace the tape every now and then, as it''ll disintegrate due to UV.