Bleaching and discoloration of wood after removal of old finish

I get all sorts of enquiries about refinishing old pieces of furniture. There are people who want to transform older pieces and put the wood to its best use. After the varnish or paint has been removed by various methods (paint stripper, thinner, sanding, sandblasting), the wood may still be covered with paint marks, stained or with pores marked because the varnish has gone deep into the wood. In this case, how can you fading wood so that it can be refinished as if the wood had nothing on top?

A very handy product is the îlaundry bleachthat is, products containing sodium hypochlorite between 1 and 5%. If it also has some caustic soda (0.1-1%) it is even better. Apply the product with a brush or cloth to the surface of the varnish-cleaned wood and allow to dry. Repeat 2-3 times for best results. The bleaching effect is enhanced by the sun. Allowing the wood to dry in the sun will make the action of the hypochlorite even more effective. After drying, the wood should not be washed with water but brushed to remove the salts left by brushing.

Bleach attacks dyes but does not attack paints or pigment-based coloring solutions. In other words, if the wood has been stained with stain, using bleach can discolor it. However, if it has been stained with paint or special pigment-based stains have been used, they cannot be removed.

It's good to know that hypochlorite doesn't attack the natural color of wood, so if you want to preserve the natural color of wood, you can use it without worry.

Another product that can be used to remove some remaining stains is oxalic acid. It is very good against water stains on wood or rust stains, marks left by old nails or hardware. To prevent damage, when treating wood with oxalic acid, the hardware must be removed. Oxalic acid is found in crystal form and is dissolved in solution in warm water. As with bleach, the solution is applied and allowed to dry, after which the salts are removed by brushing.

Oxalic acid can also be used to remove the gray color that results from the natural oxidation of wood in the sun.

If the wood also shows traces of blue mold it can be treated with perhydrol (hydrogen peroxide solution 35%) It should be borne in mind that perhydrol also attacks the natural color of the wood and discolors it. The solution is applied to the entire surface and left until the following day. There is no need to wash the wood afterwards because hydrogen peroxide leaves no residue.

A mixture of caustic soda and perhydrol 2:1. It is the classic wood bleaching mixture, presented as solution A and solution B, with instructions for mixing and use.

If you need something even more aggressive for discoloring wood, you can use perhydrol and ammoniasolution 35%. The solutions are not mixed but applied one on top of the other. First apply hydrogen peroxide, in excess, then apply ammonia. After 2 hours wash with plenty of water and allow to dry. Caution, bleached wood is finished with polyurethane products because the hardener turns yellow. In such cases it is recommended acrylic finish.

All these processes are quite aggressive. That is why you should wear gloves, mask, goggles and appropriate equipment when using the materials. Buy the materials from specialized shops and follow the instructions for use and protection exactly.