In the old days (and even today to some extent) people making latex rubber molds would go to a tire recapper and take the rubber that had been ground off of the old tires, sift it and use it as a filler for latex when making rubber molds. The advantage of using this material is that being rubber, it is flexible for filling those undercuts. We've found a supplier who is able to provide us with the same material at a reasonable cost so that we may offer it here. Great additive for Brush-On 40 and 50 as well, if you want something that is low cost and flexible.