It is very important in selection of paints to pay attention to their physical-and-mechanical and processing behavior, since they are relating to convenience of applying, speed of drying and finally to appearance of the coating. Some important properties of paints can be distinguished, such as:
Spreading Capacity – Density – Thixotropy – Spreadability – Drying Time – Consumption
Spreading capacity is the behavior of material to close the contrast substrate at certain consumption. There are materials that allow covering of contrast substrate at one pass. They are differed by higher prices, but fewer consumption. It’s possible to get adverse situation when for covering of contrast substrate (for example, the black-and-white one) it’ll be necessary to apply 4-5 coats. This means that the material has poor spreading capacity. It’s accepted that good-quality materials must be applied in two coats.
Density of paint in combination with its spreading capacity are good indicators of the paint quality. The lesser density possessed by coating preserving at the same time good covering capacity the easier to work with such material. (This is especially noticeable at painting of large areas using roller-brush: after some time the weight of roller with paint will certainly affect to the work performance of painter, thus the lighter is a paint the easier to work with it.)
Thixotropy is a property of material to increase its flowability during applying of mechanical load onto it. In practice this is indicating that good paint applied with roller-brush doesn’t flow and drips from it and forms even film on the surface and spreads good when applied with roller-brush. Drying time of acrylic base water-dispersion paints is practically the same for materials of different quality. Duration of drying is influenced by both the air and base temperature and the humidity in the room. For example, at the temperature of air and base of 20°С and air relative humidity of 65% the drying time usually amounts to 4-6 hours prior to following working. The paint consumption, as mentioned above, depends on its covering capacity. For water-dispersion paints it amounts to 150-200 ml/m2 usually.
Appearance and longevity of coating are depend on surface preparation for 60%, appropriate work with paint for 10-15% and quality of paint itself for 25-30%.
Preparation of surface
Preparation of surface prior to painting has significant importance for obtaining high-quality coat and ensuring its longevity. Preparation of surface consists of cleaning off the corrosion products, old paint, grease and other dirt. Methods of surface preparation are divided into two main groups: mechanical and chemical.
Mechanical methods include cleaning by means of tools (brush, polishing machines), cleaning using sand, shots, sand-and-water mixture. Applying these methods ensures obtaining of duly cleaned surface with regular roughness, which facilitates best adhesion of varnish-and-paint film. Chemical methods of surface cleaning include, firstly, surface degreasing, which is carried out by means of alkaline detergents or using active solvents (paint removers) depending on type of dirt.
During renewal of varnish-and-paint coating it’s necessary to inspect it thoroughly. If the old paint coat adhered fast on the surface in the form of continuous layer it should be washed with warm water using detergents and dried. If the coat is loose it must be removed completely.
The first operation after preparation of surface is grounding. This is one of important and critical operations. First primary coat serves as base for all coating. Ground serves for creating of strong bond between the painted surface and the following paint coats. It provides high protection ability of coating as well. The prime can be applied using brush, paint-sprayer or other means. Prime coat must be thin. Drying of prime coat is carried out in accordance with mode defined by the technology.
Filling is necessary for surfacing. The filler is to be applied with thin layer. Thick layers of filler can become cracked during usage resulting in decrease of protective properties of coating. Each layer of filler must be dried appropriately. Filler is applied at least in three layers with thickness not exceeding 3 mm.
After dryout of filler the surface will have irregularities and roughness. Polishing will be applied for eliminating these conditions. Hairlines appear on the surface in process of polishing and it becomes dull. At the same time adhesion between paint coats is improving considerably. Paper and fabric base abrasive cloth is used for polishing.
Enamels, paints and varnishes are applied on to the surface by means of paintbrush, roller-brush and paint-sprayer, etc.
The best time for carrying out of outdoors painting works is an early spring or autumn time, when there is an opportunity for choosing dry and warm but not hot periods of weather and with small temperature disturbances. Carrying out of painting works is not desirable at early morning because of dew. In summer it’s better to paint at second half of a day. At this time of a day sunbeams are heating not so strong, but the painted construction is warmed up. All varnish-and-paint materials designed for finishing of facades are referred to protective-decorative coatings. They are not destined for decoration of construction only, but also for abating of destructive effect of environment on it.
Main forms of destructive effects are the following:
The most powerful destroyers are variable temperatures in combination with increased humidity, pollution of air and the sunlight.
Paint and painted surface must correspond to each other, which ensures service life of coat being long and its nice appearance.
Mineral surfaces are characterized by their inherent ability to take in moisture when the temperature is decreased and to dispose of it in the form of vapor when the temperature is increased. Therefore brick and concrete paints for such surfaces must have structures that will be waterproof but will release vapors, otherwise vapors will pull the paint film off the surface. Paints restricting breathing of wall’s facades will promote accumulation of moisture within paintwork resulting in destruction of its mineral base. Old brick constructions painted with weakly breathing paints are turning into loose mass under plaster. Not all types of water-dispersion paints are suitable for concrete surfaces. They draw all alkali to the surface causing dealkalization of upper layers of the base. This brings to destruction of steel reinforcement as well as can bring to accelerated fading of the very coating.
Most important factors in respect of wooden surfaces are the elasticity and water resistance of paint. Prime importance in respect of paints destined for metallic surfaces is to prevent permeation of water to metal base, preserve elasticity for a long time and prevent from corrosion.