That is interesting. Please take some pictures of what you are describing. (I'm sure you were planning to anyway). I suppose the parting iron is "crushing" the wood to open up the seam? I think Abeking crushed a groove in the center of the plank edges when doing the planking on Concordias. The cotton caulking was put in the groove. Then, I suppose the crushed wood expanded when wet. Is that the same idea?