One potential problem with combination stones is one side can expand and contract at a different rate than the other side, to changing water content. For that reason, I don't like them.
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And, just for another datapoint, my Norton 4/8K combo stone has been sitting in water continuously since 2009. I use it on every honing job I do.
I guess, as they say, YMMV.
James.
I've heard those were good stones. This one cost me $20. Before I soaked it for months it had a very nice feel to it and did a very good job. But it just didn't feel quite the same after soaking for months.
But, regardless, my version of science thinks going back to the original successful versions is the first test. Then some day may try leaving it in the bath for longer and see.