How to avoid film gassing when spraying varnish

Unlike brush or trowel application, spraying allows a larger and more uniform amount of material to be deposited on the wood surface. Despite these advantages, there is one disadvantage that stops many people from using this method of application. This is the formation of that cloud of fine particles of varnish - overspray - which, in addition to the loss of material, is the source of other problems. One of them is gassing of the film.

Some of these varnish particles dry in the air and fall as dust onto the still liquid varnish film and get trapped in it. In the end, the varnish will have an unpleasant, whitish appearance and a rough, wrinkled streak. If the film formed is primer and not varnish, the defect is removed by sanding between layers. In the case of the final coat of varnish, this can no longer be done and the effect is all the more dramatic the higher the gloss of the varnish (the more glossy the varnish).

The speed at which varnish particles turn to dust depends on several factors. Primarily thinner used. If it is a quickly volatile thinner, varnish dust will form more quickly. This is why the choice of thinner also depends on the ambient temperature, and the use of very fast solvents (such as acetone) is not recommended when spraying varnishes.

Another influencing factor is the air pressure from the compressor. When the application is done with a hand gun the air pressure adjustment is also done manually. Often, out of a desire for better productivity (actually an illusion) there is a tendency to increase the pressure. This primarily results in increased overspray. It is good to keep the pressure around 3 atm, maximum 4.

If there is a large volume of air with which the varnish is mixed, combined with a more rapidly volatile thinner, another phenomenon, very similar in appearance, occurs in addition to the varnish powder. Surface drying occurs before all the air in the film has escaped. As it tends to come out, it pushes into the formed varnish clump and the appearance is very similar to that of dust falling on the varnish film. In this case, if the surface is lightly sanded, the surface layer is removed and the small holes left in the varnish by the air appear. To avoid this, the air pressure must not be too high and, in the case of pumps, the mixing ratio recommended by the manufacturer must be observed.

When spraying in front of a well-functioning spray booth and the application is made from the area closest to the sprayer and farthest from the booth, moving towards the side of the booth, the overspray will be picked up by the spray booth suction. In the case of dry filter booths, for good suction, the filters should be shaken very often and changed when clogged.

Applying varnish in closed areas of furniture (inside cabinets, drawer cupboards) can lead to gassing. The resulting overspray cannot escape and is trapped in the film. The result is a whitish film that scratches when you run your hand over it. This is avoided if the pieces are varnished without the back of the furniture being fitted.

High temperature, lack of exhaust, also influence the occurrence of gassing. Don't think it's only solvent-based films that gas. Hydrodilutable films are also gassed. A high temperature in the workroom, high air pressure, also causes the fine particles of waterborne varnish to dry in the air and gas the film. Therefore it is good to give due importance to the elimination of overspray and to keep a constant temperature of 18-20 grd.C in the work room.