Determining the quality level of a varnish film

When it is done wood finishing, i.e. layers of dyes, primers and varnishes, the purpose is not only aesthetic but also protective. The resulting film increases the resistance of wooden surfaces to knocks, stains and scratches. How well the film performs can be determined by determining the quality level.

Depending on the quality of the finishing products, the technology used, the compliance the indications in the data sheets and technologies, a film with certain characteristics and qualities is obtained. These can be assessed using national, European and international standardised tests. The characteristics tested are related to both the aesthetic side, such as gloss or coating, and the film's resistance - resistance to staining, scratching, thermal shock.

Tests related to varnish film aesthetics

Degree of gloss – gloss - as it is known in factories, is one of the aesthetic properties of surfaces. With its help, finishes are classified into matt and glossy. The method for determining gloss is called Gardner and is based on light reflection. Gloss expresses the extent to which light is reflected by the varnish film. Values from 0 to 100 (percentage of reflected light radiation) can be obtained by measurement. These values allow differentiation in the finish deep matt (natural effect - under 10), mat (10-35), semi-luxury (35-60), pike (60-90) and with high gloss (over 90).

Another aesthetic property is the degree of wood coating. From this point of view, the finish can be with closed pores or open. The closed-pore finish is the one that completely covers the wood pore. In this case, the film is continuous, without those 'holes' next to each pore. If the lacquer enters the pores, covering only the inner walls of the pore, the finish is open-pore. The final film will also reproduce the design of the wood, and therefore the pores will be visible.

Tests to determine the strength of the varnish film

The varnish film strength tests relate to resistance to scratching, knocking, wear, tearing, thermal changes, staining, as well as to expansion and contraction movements of the wood. Tests are carried out on standardised size finish samples, called epruvetewhich is a faithful reproduction of the finish used, with all its stages. The specimens must stand for a minimum of 14 days before testing so that chemical reactions and drying can be completed. The results of these tests are graded from 1 to 5, where 1 represents the worst resistance and 5 the best.

The Romanian standard regulating the general quality conditions of transparent and opaque finish for interior furniture is SR 6009/2006. This is an adaptation of older STASs taking into account European regulations. This standard does not apply to finishes based on waxes and oils.

There are European (EN), international (ISO) and country standards. Below are some examples:

EN 12720 Resistance to cold wet sources (water stains, coffee, alcohol)
EN 438-1:2 Abrasion resistance
ISO 4211-4 Impact resistance
ISO 4211-3 Resistance to hot dry sources
SS 839117 Scratch resistance - Swedish standard

In factories there have been and always will be simpler methods to determine the quality of the lacquer film applied. One well-known method is to test film adhesion varnish to wood. It is done with a 50 bani coin that is pulled with the edge over the film. If a white trace remains, the film has no adhesion. If no trace remains or only a light bevel appears, with no white trace, the film has good adhesion.

If you know of other simple methods that can be used in factories to determine film quality, please discuss them in the comments section.