Thanks for clarifying Andy.
Yes, you are correct, the rims have to be smoothed to allow the glueing of the foam to the rim.
As for additive, a lot of clubs do not allow additive any more and so we very commonly run foams without additive. There are other way's to tune the foam such as putting small grooves in them (as seen on the pair that DC was using).
Personally I think they are less destructive than rubber and there is a good argument against people truing the foam down so that there is only a slither showing on the rim because I think it really spoils the look of the cars.
That said, it doesn't matter now. We'll know next time.
