A binder which is a graft polymer bearing at least two alkoxysilyl groups of the formula --Si(R.sub.1).sub.m(OR.sub.2).sub.3-m in which R.sub.1 and R.sub.2 are independently linear or branched alkyl groups containing 1 to 6 carbon atoms and m is an integer of 0 to 2. The graft polymer has graft branches on a graft base. The graft polymer contains at least 0.5% by weight, based on the graft base, of graft branches.