Session 3: Surface Preparation
The quality of surface preparation significantly affects the performance, durability and quality of the job. Coating integrity and service life will be reduced because of improperly prepared surfaces. As high as 80% of all coating failures can be directly attributed to inadequate surface preparation that affects coating adhesion. Even the most expensive and good quality coating system will fail if the surface preparation is incorrect or incomplete.
The first step should always be a thorough examination of the surface to be painted; checking for peeling and faded paint, dirt, chalking, grease, dust, cracking, knots, bare areas, mildew, algae, spider webs, rust, nail stains and structural problems. All surfaces, whether painted or unpainted, must be dry, clean and free from shine prior to finishing.
Good surface preparation is important to successful painting for reasons:
- Paint adhesion: Surfaces that are in good repair, sanded and clean provide a durable foundation for the bonding of applied paint.
- Durability: Well-prepared surfaces help to extend the life of the paint.
- Aesthetics: Well-prepared surfaces look better than poorly prepared surfaces. Poorly done repairs, gaps and other surface imperfections all detract from the painting job.
- Saves time: Get the job done right the first time! Having to redo surface preparation and paint application wastes time, money and causes unnecessary delays and loss of goodwill.
Problems Associated with Various Substrates
Plaster
Contains moisture which, if painted over, especially with oil-based paints, will cause premature peeling off and at times discolouration. It also contains alkaline salts (caused by efflorescence) which attack oil-based paints, causing discolouration and peeling off (saponification). The third problem with plaster is that it is porous, thus consuming more paint than usual and making the estimation of quantities required difficult. Most plasters also have an uneven surface and this uneven profile will be more highlighted if painted with a high sheen or high gloss coatings.
Metals
Producers of metals coat them with a thin oil film to prevent rusting during storage and for easy handling. If this oil film is not removed, it will prevent paint from adhering firmly. Metals also have a smooth surface which is difficult for paint to stick. Rust may also form on metal surfaces during storage and if painted over, it will reduce adhesion and discolour some paints.
Wood
Wood is porous and thus takes excessive paint, requiring extra coats to give a good finish. Woods also contain knots from which resin will bleed, discolouring the paint. If not properly dried, the moisture will prevent paint from adhering correctly. Wood is also liable to attack by fungus or insects.
Plastic/Fibreglass
These have smooth surfaces which are difficult for paint to adhere on. There is also a problem with an oily surface which is difficult for paint to stick. Some plastics may also be attacked by solvents and thus cannot be painted with oil-based paints.
General Surface Preparation
- Dirt, grease, oil, chalk, grime: Remove these deposits by washing with a detergent using a sponge or brush, rinse thoroughly with clean water and allow to dry.
- Mildew, fungus and bacteria: Spotty patches that look like dirt, but do not come off when scrubbed with soapy water, are probably mildew. These can occur on any side of the house but are more likely to grow in shaded areas or behind shrubbery. They can easily be identified from other forms of discolouration by applying a few drops of bleach (Jik or HTH). If present, the black, grey or brown colour will bleach out and disappear within one or two minutes. These must be killed and removed before repainting. If not completely removed, the active spores will continue to grow and may almost immediately begin to reappear on a recently repainted surface. Where these are present, apply a solution of one part household bleach and three parts water, or Crown Fungicidal Wash.
- Porous substrates: Seal with suitable sealer; Crown Alkali Resisting Wall Primer, Crown Plus 3 Masonry Primer or Penetrating Primer for plaster, or Crown Acrylic Primer Sealer Undercoat for boards and Sanding Sealer for wood. Plaster can also be primed with good quality matt water-based paint if it is to be finished in water-based paint.
- Powdery/chalky walls: Seal with Crown Penetrating Primer.
- Rust: Remove with rust remover, sandpaper, wire-brushing or steel wool.
- Cracks, splits, dents & open joints: These must be filled with suitable filler like Crown Polyfilla, Gyplast, Wallcare or Sandtex Exterior Filler, allowed to dry and then sanded to a smooth surface before painting.
- Glossy surfaces: Always dull surfaces with sandpaper or wire brush.
- Loose or peeling paint: Remove as much loose and peeling paint as possible by scraping or wire brush. Sand rough edges with fine sandpaper until they blend with the bare surface and apply a sealer or primer before painting.
- Nails: Punch in all nail heads. Replace loose nails with slightly larger, galvanized nails. Prime with a corrosion-resistant oil-based primer.
- Knots: Treat with knot sealer before painting.
- Previously coated surfaces: Maintenance painting will frequently not permit complete removal of all old coatings prior to repainting a particular surface. Any surface preparation short of removal of all old coatings may compromise the service life of the new coating system. Check the compatibility of previously painted surfaces using a test patch with the coating if there is any doubt on re-coatability.
Specific Surfaces
New Aluminium & Galvanised Surfaces: Solvent clean (with White Spirit) or degreaser to remove any oily residue until a ‘water-break free’ surface is achieved. Prime with an etch primer and then Zinc Phosphate/Chromate primer.
Aged Aluminium (exposed to the weather for at least 6 months): Wipe clean from dirt and prime with Zinc Phosphate/Chromate primer.
Copper: Clean free of dirt, oxides and foreign matter. Acid-etch with hydrochloric acid to remove oxidation. Prime with Zinc Phosphate/Chromate primer.
Brass & Bronze: Remove only loose tarnish, but not all tarnish needs to be removed. Then wash with detergent to get rid of dirt and grease. Rinse well and allow to dry. Prime with etch primer.
Chrome: Roughen up the surface with fine sandpaper or emery cloth. Wash with soapy water, then rinse and allow to dry. Prime with Zinc Phosphate/Chromate primer.
New iron, steel & ferrous metal: Wipe clean with mineral spirits to eliminate any oil or grease. Remove all rust and mill scale. Prime with Zinc Phosphate/Chromate primer.
Previously painted iron, steel & ferrous metal: If the old film is in good sound condition, de-gloss the old surface with light sanding and clean with solvent. If the old film is in poor condition, it should be removed with paint remover. If metal has rusted, that too must be removed with Crown Rust Remover. Prime all bare spots with Zinc Phosphate/Chromate primer.
Stainless/Mild Steel: Wash to remove grease with a degreaser. Sand lightly to etch the surface. Prime with Zinc Phosphate/Chromate primer.
New unpainted concrete floors: Check for any dampness by placing a rubber mat down and leaving overnight. Upon inspection, if dampness occurs on the underside of the mat, or the concrete surface has been darkened by moisture – Do Not Paint. New concrete should be allowed to cure for 30 days prior to painting. Etch with a solution of one part hydrochloric acid to two parts water before painting. Allow the solution to bubble, then rinse with clean water. Brush while rinsing to remove all loose concrete. Allow to dry completely. Additional vacuuming may be required to remove powdery residue left from etching. Concrete can also be prepared for painting by lightly sandblasting. The first coat of paint should be a 10–15% thinned-down version of the finish. The topcoat should be used straight out of the can.
Painted concrete or wood flooring: Ensure surface is free from dirt, dust, etc. by sweeping or vacuum cleaning. Remove grease, oil, floor compound and wax by chemical cleaning. Scrape carefully to remove deteriorated coatings. If the remaining coating is glossy or very hard, sand it lightly for good adhesion of subsequent coatings. The surface must be thoroughly dry before coating.
Masonry (Block & Concrete): Allow to dry 30 days under normal drying conditions prior to painting. If efflorescence or cement dust is present on masonry and concrete, it should be removed by etching with a 10% solution of muriatic acid. Flush off the surface with clean water after etching and allow to dry. If etching is not possible to neutralize efflorescence; sand, scrape and wire brush, then coat with +3 Masonry Primer before painting. Surfaces should also be free of all dust, dirt and loose or excess mortar. Porous surfaces should be filled with appropriate filler before painting. No special primer is required if water-based finishes are used. For oil-based finishes, apply an alkali-resistant primer.
Unpainted stucco, brick and stone: This needs no special preparation. However, stucco should be allowed to sit and dry thoroughly before it is painted. If the surface is soft or slightly powdery, first apply one coat of Penetrating Primer then apply two coats of exterior water-based paint.
Ceramic tile and glazed brick: Wash with detergent, then roughen surface with sandpaper. Paint with a 2-pack epoxy paint.
Unpainted exterior wood surfaces: Should be clean and dry. No painting should be done immediately after rains or during foggy weather, or when the temperature is below 10°C. Knots and pitch streaks shall be sanded and spot primed before receiving a full coat of primer. All nail holes or small openings should be filled after the priming. Prime with Crown Wood or Aluminium Wood Primer.
Cedar or Redwood: The main problem with painting these is that they discolour. This is known as tannin bleed. To control bleed, prime with Crown Wood or Aluminium Wood Primer which will afford better stain-blocking characteristics. In wood containing an extremely high amount of tannin, two coats of primer may be necessary.
Hardboard: Before finish is applied to the panels, they must be cleaned. Apply Crown Wood Primer or Crown Aluminium Wood Primer, even if hardboard is pre-primed, then finish with the desired topcoat.
Asbestos: If glazed, allow at least two years before painting. If shingles are porous, treat with Penetrating Primer. If shingles are weathered, remove all dirt and dust.
Plaster: Allow to dry thoroughly for at least 30 days before painting. Bare plaster should be dry, cured and hard. Fill any holes and cracks and sand smooth. Paint with water-based paint. If previously coated with a cement-based coating or lime wash, treat first with Penetrating Primer.
Drywall, sheetrock, gypsum board: Prime with Acrylic Primer Sealer Undercoat. Do not use a solvent-based primer since it will raise the fibre of the wallboard.
Acoustical Tile: Clean thoroughly and finish with matt PVA to retain acoustics. Spraying is the best way to paint these surfaces, but brush and roll-on methods may also be used.
Wood panelling: Sand lightly and clean with mineral spirits to remove any wax. Prime with Sanding Sealer for 1K Varnishes or White Wood Primer for painting.
Interior Woodwork: Sand smooth, with the grain – not across it. Surface blemishes shall be corrected. If painting, use Crown Wood Primer as primer and Sanding Sealer for 1K Varnish.
Wallpaper: Remove loose paper. Test for bleed by applying water-based paint to a small area to make sure wallpaper is water-fast. If bleed occurs, seal paper with one coat of white shellac reduced with an equal volume of alcohol. Avoid skips when applying shellac. Allow drying one hour before top coating.
Fibreglass: Solvent clean and lightly sand. Apply etch primer. Light surface areas may only require an alkyd topcoat.
Glass: Wash and allow to air dry. Apply either an epoxy or alkyd enamel.
Plastic: If flexible, do not paint. If stiff, wash with detergent and roughen with sandpaper or steel wool. Prime with etch primer.
Preparatory Products
These are products used to prepare the surfaces prior to painting to eliminate or reduce problems encountered in painting.
- Crown Metal Putty & Okay Metal Putty: For glazing/bedding glass into metal or primed wooden frames which will subsequently be painted.
- Bondcrete: For use with sand cement renderings as a bonding agent for direct application to the substrate to increase the waterproofing, tensile strength and dustproof properties of internal and external concrete floors and plaster, repairing damaged concrete and for filling cracks in wall surfaces.
- Crown Silicone Waterproofing Solution: A transparent solution used as a waterproof barrier to prevent moisture ingress on exterior porous masonry surfaces.
- Crown Mordant Solution: A light blue acidic wash which, when applied to newly galvanized surfaces, produces a reaction which aids adhesion of subsequent paint coatings. The surfaces will blacken if the wash is functioning correctly – contaminated areas will not show any change in colour, indicating a further application is required.
- Crown Fungicidal Wash: An aqueous solution of biocide, designed to destroy and sterilize mould and algae on surfaces.
- Crown Polyfilla: A powder mixed with water 1:3 to form a thick paste for filling interior minor dents and cracks on plaster or wood. Can be mixed with 10% gloss paint to make ‘Swedish Putty’ for exterior use.
- Crown Gyplast: A gypsum-based powder, mixed with water 1:3 to provide a white, smooth and matt finish on internal brick, block and concrete.
- Crown Wallcare Putty: A water-based, ready mixed thick material for smoothening/skimming interior and exterior uneven plasterwork, block boards and timber surfaces before paint application.
- Crown Paint & Varnish Remover: A corrosive, colourless dense liquid applied by brush or spatula for removing paint from metal and wood.
- Crown Rust Remover: An acidic colourless liquid for removing rust from metal surfaces before painting.
- Crown Thinners: Other than thinning paint, White Spirit can also be used to wipe off oils and greases from metal and plastic surfaces before priming.