[Ken Mason]

Not sure. I was always told sponge tyres would destroy any grip you have put down on your track from rubber tyres, which is why you should stick to one or the other. I have been to the Silverwood track at Long Sutton where they normally race metal chassis with sponge tyres and put goop down on the track first etc. 

My hard body cars with rubber tyres wouldn't move till the goop was wiped off!