La tecnologia

Before starting a varnish system, it is also necessary to check:

The suitability of environmental factors:
- the degree of humidity must be between 40% and 75%;
- the ideal environmental temperature is between 18°C and 22°C;
- the environment must be free of dust and other possibly polluting elements (silicone products, etc.)

The suitability of the support to be varnished:
- the support must have a degree of humidity of between 8% and 14%;
- the temperature of the support must be between 18°C and 22°C;
- the support must be adequately prepared (gauged, sanded, dusted, cleaned).

The suitability of the chosen products:
- the suitability of the product for the specific use must be checked, consulting the technical sheets and making precautionary tests; it is particularly important to check that the varnish system is suitable for obtaining the desired result;
- for special supports (for example paper impregnated with exotic essences) it is very important to check the suitability of the varnish products that you wish to use with respect to their adherence to the support;
- the products used must not have been deteriorated;
- the temperature of the varnish products must be checked; at the moment of application the temperature of the environment must be between 18°C and 22°C.

The correct use of the varnish product:
- only the catalysers listed on the relative can and technical sheets of the product chosen must be used, and in the percentages indicated;
- the product must be diluted with a suitable thinner and in such a quantity as to obtain the correct viscosity for application;
- to avoid unforeseen problems (cracks, colour fading, drying problems, etc.) the varnish systems and application quantities per square metre indicated on the technical sheets should be followed.

The correct use and functioning of the application/drying equipment:
- check that all machines used for application function correctly;
- check that the environment or the drying tunnel have the correct temperature required for drying;
- check that the auxiliary machines used (sanders, smoothers, polishers, etc.) function correctly;

To check that the conditions for correct varnish are respected, the following instruments are necessary:

- Technical sheets, to check conformity to standard and the suitability of the products used;
- thermo-hygrometer, to check the suitability of the environmental factors;
- wood hygrometer, to check the suitability of the wooden support (as regards humidity);
- thermometer - viscometer - weighing machine - chronometer: instruments to measure the different values of the varnish products and to check conformity to the indications on the technical sheets.

The type of support to be varnished:
Apart from solid wood, the following supports are also used, according to the type of finish desired and the costs to be respected:

- raw chipboard
- chipboard veneered with various types of wood
- paper coated chipboard with various degrees of impregnation
- chipboard coated with composite veneers
- plywood
- multi-layer
- extremely thin particle layers (under brand names such as: faesite, masonite, ledorex)
- panelled
- medium density
- cane, wicker, rattan

The varnished products and the most suitable cycles must be chosen according to the specific characteristics of the support.

Types of finish:
Finishes are classified according to three main characteristics: pore pattern, degree of opacity, degree of cover.

With regard to the capacity of showing the pores of the wood, a finish is defined as:

open pore

closed pore

To obtain an open pore finish, undercoat and topcoat varnished products with a low dry residue must be used, and for a closed pore finish, products with a higher dry residue.

The same varnish system can produce different finishes on different woods, due to the different structural characteristics and the different absorption capacities of the different kinds of wood.

Woods with particularly deep pores, such as ash, oak, or chestnut, are enhanced by an open pore finish.
For woods with a porosity that is not very evident, such as maple, birch, pine and beech, however, only more or less closed pore varnish systems are used.
With regard to the degree of opacity, a finish is defined as:
- matt
- semi-matt
- eggshell
- gloss

The same finish can have different degrees of opacity, depending on:
- the base on which the topcoat is applied;
- the amount of varnish per square metre applied, both as regards the undercoat and the topcoat;
- the degree of dilution and the type of catalysis;
- the drying conditions foreseen for the finish.

With regard to the capacity of covering the support, a finish is defined as:

transparent

pigmented (in any colour)

Apart from the main characteristics, a finish is distinguished by the specific chemical, physical or aesthetic characteristics given to the support and by the possible application systems.

Application systems

The most commonly used systems for the application of varnish products in the wood sector are:

application with a brush, cloth, sponge or tampon
used in the artisan and do-it-yourself sectors;

application by immersion
mainly used for:
- lathed objects;
- door and window frames (for impregnation, and in some cases also for base and topcoats);
- chairs (for being painted).

spray application:
spray application foresees the application of the nebulised varnished product onto the support by means of a special gun.
Spray application can be:

by pneumatic spray:
the nebulization of the varnished product is achieved by the action of compressed air through the gun, fuelled by:
- a reservoir placed above (by gravity);
a reservoir placed below (by aspiration);
- a separate reservoir hermetically sealed by the action of the compressed air;
- a separate reservoir by the action of a pump.

by air-free spray:
the nebulization of the varnish product is achieved by the action of a high pressure pump, without air;

by airmix spray:
the nebulization of the varnish product is achieved by the action of a medium pressure pump, with the introduction of air sucked in lightly;

by electrostatic spray:
the varnish product, charged electrically inside the spray gun, is drawn towards the support to be varnished by the action of an electromagnetic field created between the applicator and the support.
This system, which is particularly effective for cutting down the waste of product, is used for varnish supports with a rounded shape.

For varnish products constituted of more than one component, when problems of pot-life can arise (polyester and polyurethane type), the guns can be fed by special pumps for bi-component products, which take the two components from two separate reservoirs, mixing them in the spray phase:

Drying systems
The most commonly used varnish drying systems in the wood sector can be divided into two categories:

- natural drying
- forced drying.

With natural drying, the evaporation of the solvents and/or the chemical reaction of the products applied, which lead to the hardening of the varnish film, occur in the air.

With forced drying, the evaporation of the solvents and/or the chemical reaction of the products applied, which lead to the hardening of the paint film, occurs in a tunnel where there is forced air circulation and the complimentary use of further energy sources, such as:
· hot air (the tunnel can be of the carousel , single layer, multiple layer, vertical, or hanging rack type);
· radiations emitted by special lamps (tunnel with infrared and ultraviolet lamps).

The formation of a varnish film can be accomplished by means of:

physical drying
The film of a varnish product forms subsequent to the simple evaporations of the solvents and thinners, without any chemical modification of the binder. Al the single-component products dry in this way (for example: nitro-cellulose, vinyl and water based products, etc.)

Chemical drying:
The film of a varnish product forms subsequent to the chemical modification of the initial characteristics of the binder, associated with the formation of the film due to the evaporation of the solvents:
by chemical reaction between the links contained in the single components.
The polyurethane products with two components dry in this way due to the effect of catalysers and accelerators.
The polyurethane and urea products dry in this way by the effect of particular UV radiations: the UV radiations cause the decomposition of the photo-initiators contained or added at the moment the products are used.
Polyester products and UV fillers dry in this way

Drying by oxidation
The film forms subsequent to the action of the evaporation of the solvents and the slow reaction of the binder with the oxygen in the air, which modifies the initial chemical structure of the binder.
Synthetic oil, flatting or oil-based urethane products dry in this way

Sanding
In a varnish system, sanding can be carried out by the following systems:
- manually
- with semi-automatic machines
- with automatic machines with several layers

Sanding must always be carried out in a direction parallel to the direction of the fibres of the wood.
It must always be carried out:
- on the raw support, before beginning the varnish system;
- on the surface varnished with the basecoat, before applying the finish.

It may be necessary, in order to achieve adequate adherence of the varnish product to be applied on top, to carry out a further sanding operation:
- between two polyurethane or polyester basecoats, if the maximum time limit for the second coat has been exceeded
- between the application of an insulation, if the maximum time limit for the application on top of the basecoat has been exceeded

Sanding of the support:
- To achieve closed pore varnish, sanding the support with 120-150 grain paper is recommended.
- To achieve open pore varnish, sanding the support with 150-180 grain paper is recommended.

Sanding the base:
- For the sanding of a sprayed polyester base, the use of sandpaper with a 220-240 grain is recommended (1st strip) as a first step and then with 320-380 paper (2nd strip) before the application of the finish.

- For sanding a polyurethane base, the use of sandpaper with a 280 grain is recommended (1st strip) as a first step and then with 320-360 paper (2nd strip) before the application of the finish.

- For sanding a nitro-cellulose or synthetic base, 220-320 sandpaper is recommended.

- Special needs may in any case require the use of solutions other than those indicated.

For sanding profiles such as poles, frames, baseboards, matchboarding, etc. special sanders are used, specifically designed for such elements, equipped with strips of sandpaper with counter-profiles or specially shaped discs, of various degrees of surface hardness (scotch-brite type).

Polishing systems
To achieve particularly glossy and brilliant finishes (mirror finish), in the final phase of the varnish system, special polishing machines can be used, constituted of one or more rollers, each composed of several felt disks fitted on a shaft, kept in rotation by a motor, while the panel to be polished passes underneath.

Polishers can be constituted of:
- a single roller
the panel to be polished passes underneath the polisher several times; the surface on which the panels to be polished are places travels backwards and forwards under the roller;

- of a series of rollers
the panel to be polished enters on one side of the polisher and exits from the opposite side; this system is commonly used in industrial mass productions.

For the polishing of limited productions, orbital hand polishers may also be used.