The old, loose paint layer have to pickling by burning, completely removed with a wire brush or sandpaper. To skimp on work steps, you can immediately treat the wood with a paint remover e.g. Brillux Top 155. After the agency of the paint remover the color can be deported/removed with a spatula. To remove remaining leavings from cracks and pores you can use a high-pressure cleaner to wash out. Before continuing work of the substrate should be tight, dry, clean, viable and free of release agents.