Pickling has two meanings. Firstly, it is a procedure to give wood an attractive appearance. An additional advantage here is that staining gives protection against moisture. There are water- and oil-based stains. All types of wood can be stained. It is important to prepare the wood properly by sanding it smooth and making it dust-free. Stain can be applied with a brush as well as with a cloth. Secondly, pickling is also the name for treating metals with (weak) acids. Pickling deeply cleans the surface and removes, for example, mill scale. Pickling is therefore done in preparation for applying an additional surface layer, or to bring out the crystal structure better for microscopic examination.