Well nobody at HD knew what the sealant is. It's too tacky to be joint compound. I put some on my fingers to test it. The guy at HD said it reminds him of silly-putty or that 3M sticky tack that you use to hang posters. So, I just scraped as much off as I could with a plastic putty knife. I actually had to do that regardless because the big pipe wouldn't go back in with that stuff on there.
So, after a couple trips to HD, I ended up finding this thing with the other ferncos.
It was called a transition, and the bigger end fits over the larger diameter of the 90-elbow, and the smaller over the pipe. Looks like it was made for exactly this purpose. Here's some pics of the finished install:
Now for the leak test. I don't mind if the grey water drips, but I'm hoping I get lucky with the black water connection since I didn't use sealant.
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