You can never overdo it with WD-40, cardboard, paper, etc... They are all carbon based organic materials.
If I ever have any voids, pinholes, etc.... I can almost always trace it back to not banging on the can long enough. I build a can and add the powder, then bang on the side for 10 minutes, and then do it again and again until it has at least 30 minutes of banging.
I think using large flat dies are the way to go. I never use squaring dies. Get it that yellow going to white color. I make sure everytime I flip the billet in the forge that I don't do the initial weld until the color underneath the billet is the same as the rest of the forge.
Just bump it on one side, spin it 90 degrees and bump it again and toss it back into the forge. Wait until it gets the same temp and do it again, and as many times as it takes until you don't feel any spongyness. Then reduce.
make sure you don't do too much per heat. Its better off to be patient.