A washcoat is a very thin layer of paint or sealer. Washcoats are used in the preparation of catalytic converters and also in the staining of wood products like cabinets and other furniture. The composition of a washcoat varies depending on where it is being used. Home supply stores usually carry ingredients for woodworking washcoats, while the materials used in catalytic converters are proprietary to the manufacturer.
In the case of a catalytic converter, used to treat exhaust to reduce pollution, washcoats cover the core of the device, with the catalyst held in suspension in the washcoat. The exhaust is vented through the catalytic converter, where a chemical reaction takes place to convert some of the toxins into safer chemical compounds. Catalytic converters were once widely used on vehicles, but have since been replaced by other systems and they can still be seen on tractors, generators, and a variety of other devices.
Washcoats for woodworking are applied before wood is stained if there are concerns that the stain will become blotchy or patchy. For this type of work, the wood is first sanded smooth. A washcoat is applied and lightly sanded before the stain is added. The partial sealing will help the wood resist the deep penetration that results in spotting and blotching, yielding a finished product with a crisper, cleaner look. Other treatments, such as another sealer to protect the wood, can be applied after the stain.
Woodworkers have to mix the sealer before use. It must be diluted so that the wood can still take stain, but it cannot be so dilute that the stain is allowed to move right through the washcoat and into the wood. Since every wood is different, some woodworkers experiment with different dilutions and test patches on scrap wood from the project. This helps them decide where to apply the washcoat and how heavily to dilute it.
Sometimes washcoats are only necessary on the end grain, as it tends to absorb stain more readily. Since the endgrain can be highly visually interesting, woodworkers want to avoid obscuring it with stain that penetrates too deeply. In other cases, all of the wood including the end grain needs a thin coating to prevent overstaining. Woodworkers can decide on the most appropriate application on the basis of their experience with a given wood and the aesthetic goals that they are trying to achieve with the finished product.