Surface Preparation

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4b :Subrate CharacteristicsAugust 2004The texture of concrete surfaces does not effect the selection of paint type but will affect thequantity of paint required to cover the surface.Plaster Set plaster and plasterboard surfaces used internally generally show very few paintingproblems if good practices are employed, and jointed areas are smoothly finished. Paperfacedsurfaces should not be scuffed up by sandpaper during finishing as this gives a coarse unevenappearance when painted.(iii) FERROUS METALSWhile steel in its various forms is usually used for its structural strength it does need to beprotected from atmospheric pollutants, water, and other chemicals which cause corrosion. Oncestarted, corrosion is a self-perpetuating process and can quickly lead to serious weakening ofstructures and building fabrics, as well as unsightly rust stains on concrete or coated areas.It is therefore important that one of the elements in protecting iron and steel products fromcorrosion is to prevent contact with these environmental chemicals by the use of Barrier orProtective Coatings.Other mechanisms against corrosion are:Galvanic or sacrificial protection of another metal such as zinc cathodes or using chemicalpassivates such as chromates and phosphates.Two main types of steel are usually encountered although there are of course many more. Theseare COLD-ROLLED and HOT-ROLLED STEEL. Each of these need different surface preparationdue to their different manufacturing processes.HOT-ROLLED STEEL is produced with a thin layer of oxide (mill scale) on the surface whichadheres tightly when new but becomes easily detached on weathering. It is essential that this isremoved for long-lasting coating performance. Mill scale can be removed by HAND or POWERTOOL CLEANING or by BLAST CLEANING, the latter being the most effective method. Blastcleaning is done by mechanical shot-blasting, pneumatic abrasive blasting or wet abrasive blasting tospecified standard blast-cleaning grades (Class 1, 2, 2.5 or 3).COLD-ROLLED STEEL and WROUGHT IRON on the other hand are generally free from millscale and only requires the removal of oils or rust deposits by the above methods.The cleaned surfaces should be primed as soon as possible, within a few hours, to avoidrecontamination and to avoid flash rusting from moisture in the atmosphere, then top coated withthe coating appropriate to the end-use and degree of exposure.(iv) NON-FERROUS METALS(Hot Dip galvanised and Zinc Thermal Sprayed Steel, Aluminium, Zinc, Copper, Brass, Lead, etc.)These metals are not used for general structural purposes but for cladding, decorative effects, orprotection of steel substrates. Zinc (galvanising) is generally employed as a coating on steelworkby electrolytic or molten deposition. Both zinc and aluminium and their alloys are used in metalspraying where the steel surface is sprayed with molten metal.Although these non-ferrous metals do not “rust” they do corrode in normal, industrial or marineenvironments.4–14
4b :Subrate CharacteristicsAugust 2004The corrosion by-products, oxides, act as a protective layer and prevent further corrosion, but mayimpair adhesion of paint coatings to the surface.In areas where chemical attack may occur some of these metals should not be specified as theymay be attacked more severely than specially selected steels.Special Problems of Non-Ferrous Metals:Galvanised <strong>Surface</strong>s: New galvanised structural steel surfaces may have flux or other meltbathcontaminants present which need to be removed. Oily galvanised surfaces will need to bedegreased before priming. Zinc or galvanised surfaces should not be painted with alkyd paintsdirect to metal as zinc embrittlement of the paint will occur and early failure by peeling willresult. Always use appropriate primer before painting. Clean new and old galvanised surfaces wellbefore painting.Aluminium, Copper, Brass, Lead, and Chrome: While not normally painted, these metals do notpresent problems in painting unless especially smooth and / or when new. They should be cleanedand finely abraded before coating.(v) PLASTICSGenerally such components are best not painted and are usually manufactured in the end-usecolour. Painting is only required for decorative purposes where blending into existing colourschemes is desirable, e.g. downpipes and guttering.In these exterior applications paints can inhibit solar degradation of plastics, although the use ofinherently weather resistant grades of plastic is preferred.Painting of plastic items can be difficult due to surface tension effects and the inability of sometypes of paints to “wet” the surface. Textured plastic surfaces are generally more readily paintedthan smooth ones.It is very important to specify carefully or know the chemical nature of the plastic as this caninfluence greatly its performance and the paint system required. Many plastics are very difficultto paint without specialist preparation (polyethylene, polypropylene, fluoro plastic, plasticisedPVC). Others are sensitive to some solvents (polystyrene, polycarbonate, perspex). Some of thesehowever can be painted with water based paints quite successfully.Plastic surfaces cannot always be readily abraded as scratch marks often show through the thinpaint film. Oils and mould release agents must be carefully removed.Generally, with conventional surface preparation adhesion of paints to plastic can only be regardedas moderately successful.4–15
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