Metal brushes should no longer be used for applying structure on Nordic skis. For this purpose. We now have stonegrinding and hand structure tools. Brushes are to remove wax and to "open" the base. For this reason, a very soft metal brush is necessary. The softest metal brush on the market is copper. It does all of the good things that a coarser metal brush would do without the negative things that go with it (hair!). When a base has hair on it and a hot iron passes over it, the hairs melt and "seal" the base. So a hairy base doesn’t just glide slower, it also leads to long term damage of the ski base.