Questions for the painters...

In my experience, do not use Slick Sand over DP. I shot an engine compartment with DP90, came back a week later, scuffed it, and shot Slick Sand. It softened up the DP, and wouldn't cure. I had to sand the entire engine compartment down again, and just shot it with Slick Sand. They make a product called Feather Fill, with all the properties of Slick Sand( high build, easy to sand) that was created for just this reason. That is what I would use over Dp.
I would guess that there was more tot hat than product failure. Probably top coated the dp too quickly or possibly a inaccurate mix with 401 or 402. There is an induction time with 401. Dp epoxy never gets hard. It is somewhat soft by nature and that is why it is labeled a non sanding primer.