Bleaching and discoloration of wood after removal of old finish

I get all sorts of questions about refinishing old pieces of furniture. There are people who want to transform older pieces and bring out the value of the wood. After the varnish or paint has been removed by various methods (paint stripper, thinner, sanding, sandblasting), the wood may still be covered with traces of paint, stained or pore-marked because the bath has gone deep into the wood. In this case how can fading wood in such a way that it can be refinished as if the wood had nothing on top?

A very handy product is thelaundry bleach, i.e. 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 leave to dry. Repeat 2-3 times for best results. The bleaching effect is enhanced by the sun. If you leave the wood to dry in the sun the action of the hypochlorite will be even more effective. After drying, the wood is not washed with water but brushed to remove the salts left by brushing.

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

It's good to know that hypochlorite does not attack the natural colour of the wood so if you want to keep the natural colour of the 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 as crystals and dissolves in warm water to form a solution. 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 grey colour resulting from the natural oxidation of wood under the sun.

If the wood also shows traces of blue mould it can be treated with perhydrol (hydrogen peroxide solution 35%) It should be noted that perhydrol also attacks the natural colour of the wood by discolouring it. The solution is applied to the entire surface and left until the next 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. This is the classic wood bleaching mixture, presented as solution A and solution B, with instructions for mixing and use.

If something even more aggressive is needed for wood staining, you can use perhydrol and ammonia, solution 35%. The solutions are not mixed but applied on top of each 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's why you need to wear gloves, mask, goggles and appropriate equipment when using the materials. Buy the materials from specialist shops and follow the instructions for use and protection exactly.