I will remove the slots by copying and pasting slices of the image over the slots, and then use a blend tool in photoshop to hide this edit. When this gets run through the watercolor conversion all evidence of it being a slot car is removed.
Here are a couple of more.
A Lotus 40 and a Ferrari 275 P


Good to see the Porsche GT3 again. That is the car that kicked off this new livery.
Cheers
Dan