Are these tires soft?
What paint did you use?
The last Image of the tire in the vise doesn't identify wheather it's Resin or not. All of the Resin parts I've ever used are not as soft as the one in the image. The only Aftermarket tires I've ever purchached are made of Black RTV and looked like modern Radials. They're for Light Trucks, SUV's and the like.
I prefer to build my kits with modern tires. Bias Ply had their time. You'd be surprised how much better my '57 Nash Metropolitian Steered, Rolled and Handled because of them. The same was true for my '69 Austin America, '73 MG Midget, '74 Ford F-150, my '71 Toyota Corona MkII, the list is too long. Once the Radial Tires were put on, they were better than new in Ride, Handling, Traction and etc..
I've got an AMT Ford C-900, a Lindberg Dodge L-700, an AMT GMC Astro 90, and my (purchashed used/built and rebuilt by me) Ford F-8000 Toter Truck "Little Mule". These Rigs could look better with these modern Tires and Wheels.
I case you need a laugh I made a Video of the Mule- https://www.youtube.com/watch?v=6OUljJUMzNo. No thanks to anyone on or accociated with YouTube.