Several things are happening here. There's multiple things that you need to address:
- The core of all these problems is that the wood is drying too much too quickly. Place the wood in a more humid room or a colder area.
- I don't think you sealed the ends well enough. You need a real moisture barrier on the end. A coat of poly will not suffice. I use two or three coats of white wood glue myself, but several coats of poly should work as well.
- The wood is too big, in my opinion. A half log 6" x 3" will take about one and a half year to fully dry. I would have split it in quarters. That dramatically reduces drying stress.
- Why have you cut the checked ends off this early in the drying process? The wood is basically still green after two weeks. If you continue to cut off the ends every two weeks, you'll end up with a pile of 2" off cuts and no staves left... I never cut off the checked ends prior to roughing out of a fully dried stave. There's simply no reason to do it earlier. Some end cracks can be avoided in a smart bow lay-out or strengthened with a tip overlay.
- ajooter suggested wrapping staves in saran wrap to slow down drying. I've also read that before, and in principal it is true. But in my opinion, a plastic such as saran wrap does not slow down moisture loss, but it completely stops it. This will give fungi a nice environment to grow. Instead, I would advocate to wrap the staves in something that will act as a partial moisture barrier. I'm currently trying newspaper. I've wrapped some laburnum staves in newspaper and check them every two weeks for fungi or drying checks. So far so good.
