Polyglas tires are not running clean
I installed reproduction Goodyear Polyglas F70-15 tires a few months ago on a 1969 Z/28 that is mostly stock. The suspension is very fresh and I believe the alignment is good. During each drive on the new Polyglas tires, the bottom half of the car body is getting filthy - like a very fine layer of black coal dust is being deposited on the paint. The dust seems to be distributed evenly at all four corners - it's not any worse in the front than the back. It looks like a transparent haze on top of the paint, but then it leaves black soot on my fingers when I wipe those areas clean with my hand.
At first, I thought it was just the mold-release wearing off the tires or something like that, but things have not improved after 300 miles on them. I have tried raising the tire pressures: 32PSI, 37PSI, 42PSI, and I have also tried driving more gently, but nothing seems to help.
Before switching to the Polyglas tires, I was running an older set of Goodyear Eagle ST radial tires with zero problems. The body stayed nice and clean even when the car was driven aggressively.
At this point, I'm just about ready to remove the Polyglas tires and order up a new set of Radial T/A's, even though I really like the look of the Polyglas tires and I think they handle reasonably well for my needs.
Any suggestions for me before the Polyglas tires come off the car?