The P sheets for the old high end DP40 epoxy primer listed a mix ratio for using it as a sealer coat, as I remember 10% maximum and was same reducer we used with Delstar top coats. The newer lead free version DP40LP does not, or did not list same usage. I have not looked recently since I switched over to MP170. While it is not listed on the P-sheet I have reduced MP 170 in a similar manner using MR180 series reducers with great results. You must be careful to not get carried away with the reducer, I never go over 1 part in 10.
The worries with recoating epoxy primers are not in doing it too quick but in waiting too long. In reality there is no idle wait time between priming with 170 and top coat. By the time you get your primer gun rinsed and a batch of top coat mixed and in a gun you can right at applying it. For best adhesion on sheet metal and in hot weather you need to pay very close attention to the P-sheet recoat window however there is no need for panic if the job goes a bit long, meaning a day. For castings you can stretch it out a few days and will have zero problems. Cool weather also allows for stretching it some. In any case if it has set for weeks you dont recoat it without prep work.