MULTI COATS of PAINT SYSTEMS

Post -by Gautam Shah  

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A surface of an industrial product, building, roads and our body also needs a coating. It could be single layer system or a multi-layer method. A single layer system due to its simplicity and frugality may not be very superior to multi-layer method. If the ground or the sub-surface is adequate on its own, perhaps a single layer system may suffice. In case of multi-layer method, each of the coats has a distinct function. The sequential layering of multiple coats offer unique functionality. 

Technical coats operate with either the substrate or the environment. The substrate needs sealing (filling up of pores), levelling (smoothening the surface or endowing new texture), binding (of loose particles) and preparing a receptive surface for the next set of treatments. Technical coats nominally are not required to offer any sensorial qualities.

Technical coats are for Waterproofing, Electro-stat proofing, Spark erosion or proofing, Fire resistance, Mildew and Fungus proofing, White ant resistance and Rust inhibition.  

 PRIMER COAT  

A primer or sealer is the first coating applied to the objects’ surface, so it is required to:

• adhere to the object’s surface and create a suitable substrate for the next coat.

• regulate moisture movement in case of wood or masonry surfaces and provide corrosion resistance on metals.

• regulate the PH value and galvanic activity of the surface.

•  seal the surface so that oils, waxes, gases, vapours, salts and other reactive exudation from the object mass do not leach out in adverse condition.

•  fill up micro pores and crevices to level out the surface.

•    provide temporary protection to the substrate from actions like abrasion, oxidation, sparking, ignition, insects attack.

Since primers are more or less technical coats (coats whose visual value is not important), may not be coloured or tinted to any specific shade. However, in case the primer also functions as an undercoat, it may be pigmented for a specific shade. It is desirable to have primer of a slightly different and lighter colour shade than the subsequent coat, to differentiate a freshly coated surface and un-coated surface. Good primer must remain adhered to the surface, even after other top coats are removed accidentally, or for renovation.

A great variety of primers are available in the market, but of following basic three categories:

Wood primers generally function as sealers, so have a high pigment + extender, ratios. Wood sealers for clear coats are colourless coatings that help in sealing the grain. Most versatile wood sealer is NC lacquer. Commercial wood primers for pigmented paints are pinkish in colour due to the presence of white pigments and extenders, compared to metal primers (red-oxide or Zinc rich) which, are dark coloured.

Metal primers have rust inhibitive pigments or extenders like red lead, zinc oxide, zinc chromate, red oxide, calcium boro-silicate, barium meta-borate, zinc molybdates, chromium fluorides, basic lead silica chromate, zinc ferrite, calcium ferrite. Under water (submerged) metal structures are coated with zinc rich primers based on epoxy, polyurethane systems or chlorinated rubber paints are used.

Masonry primers are applied to alkaline surfaces, so are designed as non-acidic mediums. Often such surfaces have high degree of loose particles, so Masonry primers have high proportion of binding materials. Commercially these types of primers are known as cement primers and are available as water or oil-based formulations. Water-based formulations are mainly used on virgin masonry surfaces.

UNDERCOAT 

 An undercoat is an intermediate coat, single or many layers of it are required to provide:

• adhesion to both primer/sealer and topcoat

• helps achieve required film build

• provide the colour tone or true tones, if the top coat is translucent or transparent

• filling and levelling properties

• opacity

• gloss holdout

In two coat systems the undercoat may be a primer coat. An undercoat could be a specifically devised coating system, or of the same material as the top coat.

TOP COAT

Coatings begin with a primer coat on a masonry surface like plastered faces. On cement plaster surface cement primers (water thinnable polymers) are used. These primers are applied with horizontal strokes so as to receive the next treatment, the putty coat. A surface levelling putty coat is applied by vertical strokes. After sandpapering the dried putty surface, an undercoat (thinned material of super or final-coat) is applied, usually through a brush by horizontal strokes. If the job is large, the undercoat material is purposely prepared with little lighter shade than the final coat. This helps in differentiating the coated surface and the un-coated surface. Final-coat is always applied by vertical strokes of a brush or vertical rolling of a roller. 

Final coat application often incorporates some type of rendering (finish-texture). Water based interior coating systems invariably create a matt to satin finish, and to enhance this effect rollers are used for final coating and / or rendering. Rollers are made of polyurethane foam, rubber, Teflon, coir, wool, engraved acrylic, copper, etc. For oil-based glossy finish, a coat thinned with an appropriate solvent is applied by vertical strokes. Overlapping of edges is done within 20 minutes, alternatively coating is terminated in alignment with a strong architectural element like a turn, a corner, groove etc.

Some specific coatings like, lacquers require mild buffing with a Carborundum paste, after the coating. Silicone oil emulsions are also applied to create a smudge or tarnish-free surface. Final coats must be protected from fast movement of air that may cause uneven drying. Dust proofing and direct sunlight protections are obvious needs. Coatings applied very early (3 to 6.00 A.M.) do not give satisfactory finish due to high moisture changes in atmosphere. Coatings during pouring rain or soon after it pose similar problems. In dry desert climates, ideal time is past 4 P.M. Nevertheless, the surface coated should have temperature + or - 5°C. compared to an interior wall. For top coat, the coating material must be used from the same packing, executed using the same brush or roll, and preferably by the same person.

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