I have turned about 6 pens from honeycomb blanks I have made myself. One of which I turned down to a slimline, which I think was almost a dumb idea, even though the pen ended up very pretty. I do not clean the aluminum before pouring. I do use alumilite and a pressure pot. I also use a carbide roughing gouge and only take off a tiny bit at a time. I go into these blanks knowing that they're going to take a while to make. I stop roughing when I'm about 1/8 of an inch out, or if there's chunks flying everywhere, I'll stop sooner. Then I take off material with 80 grit sandpaper. Like I said, it takes a while. With this method, I'm able to consistently do bigger pens with no blowout. But with the slimline I found that even with the gentle removal method I could still see separation, so I had to part off the ends and glue everything again. I probably could have prevented it had I put on some thin ca halfway through like dieseldoc said. I usually ca the ends after trimming them.
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I think alumilite helps, but you have to be really cautious on the removal as well.