What the Heck is a Ford #6 and # 8 Viscosity Cup?

I am "testing" a waterborne Poly Topcoat and the TDS says "Viscosity using Ford Cup #6 40 +/- 5" I have googled a conversion chart for a ford #6 to #4 and found nothing. I cant even find a place to buy a number 6 cup lol.

I did thin the product 10 percent and am able to spray beautifully with my 1.8 Fujispray, but i am just curious if there is a conversion chart?

Thanks..

I don't believe you'll find a perfect conversion chart as different sized cups work for different viscosity ranges. While there is some overlap between successive cup sizes, there isn't much. Moreso when skipping a size between cups. Having said that, searching for viscosity cup conversion chart turns up many hits that might be helpful to you.

I've always used Zahn cups. But this will tell you all you want to know about Ford cups: https://en.wikipedia.org/wiki/Ford_viscosity_cup

Thanks... That clears things up :-)