HOK used to have a dedicated reducer for it's epoxy primer (I think it is EP-3). They have discontinued it, but I like it better than their new recommendation. Now they say that their regular RU series of reducers will work, like lownslow said. I like the EP-3 better becuase you need to clean the gun afterwards with the same reducer you use in the primer, lacquer thinner doesn't do it well. The old EP-3 really cleaned it out good, the RU-311 doesn't do as good a job. See if anyone's got any EP-3 laying around!