On steel square tube I've clamped wood in place,drilled through wood and metal then driven self tapping/threading screws in near bottom and top of column. Install trim to cover screw heads. On heavy wall round pipe,mig weld Simpson Strong Tiee to pipe then fasten wood to Strong Ties with self drilling screws. If wood is to be painted and no trim desired, countersink screw heads then caulk/putty.