Here is a tip for putting silicone on foam tires.. the finger way will never be perfectly smooth.. Way back in the day we use to run hard packed clay oval.. we use to silicone our foam tires.. this is how we did it..
First you need a drill that can be set at a super low speed.. almost a crawl.. very very very slow..
then you need to make a chuck, that will it the drill, and allow you to mount the tire to the drill..
Put the tire in turn it on.. the speed again should be a crawl..barely moving.. Then take a basball card, or playing card and put some silicone on it.. Apply it to the foam tire in a nice even manner.. keep spreading it on the tire until it is perfectly smooth.. let the tire spin in the drill until the silicone is dry.. the rotation of the drill will keep the silicone balanced on the tire until it is dry..
when you are done you have a perfeclty smooth round tire..