Preparation of Surfaces
For any compound to adhere properly to a substrate the surface needs to be correctly prepared. Such a preparation must produce a surface that is CLEAN and SOUND and unless a moisture compatible product is being used, the surface must also be DRY prior to the compound being applied.
Definition of Terms
Clean: means that a surface must be free of all foreign matter e.g. oil, grease, wax, old paint, dust, debris, shutter release, curing membrane and any other contaminant originating from outside the surface.
Sound: means that a surface must be free of all unsound matter originating from the surface itself e.g. millscale, rust, laitance, loose particles, white rust, oxide layers.
Dry: means that a surface must be:
- Free of ponding and surface moisture.
- Free of moisture rising through capillaries in the surface.
- At equilibrium with the atmosphere (applicable to porous surfaces such as concrete, plaster, wood etc.).
- At a surface temperature at least three degrees above the Dew Point.
To free a surface of foreign matter it may be, depending on the type of surface and nature of contaminant:
- Washed with detergent followed by thorough water washing to remove residues.
- Washed with a surfactant treated solvent such as a.b.e. super brush cleaner, followed by thorough water washing to remove all surfactant residues.
- Stubborn oil and grease deposits, as on a floor, can often be successfully removed by saturating sawdust with the solvent, applying it to the area involved and leaving it to absorb the contaminant; cleaned with a paint remover – do not use a wax based remover as this causes further problems; steam cleaned; or vapour degreased – a typical system being Trichlorethylene.
If none of the foregoing remove the contamination, it will be necessary to do this mechanically by means of:
- Wet or dry abrasive blasting
- Water blasting (high pressure washing)
- Scabbling or chipping.
Dust and debris should preferably be removed by vacuum cleaning rather than by sweeping or compressed air blowing which tend to redistribute contaminants rather than remove them. Fungus contamination must be completely removed. Scrub thoroughly with detergent and water – bleach any residual staining with a proprietary hypochlorite bleach – wash under running water – finally wash with a fungicidal solution e.g. 10% Dettol. Allow to dry.
To free a surface of unsound matter originating from the surface itself, it is usual to use mechanical means. These include:
- ABRASIVE BLASTING (grit or shot) through an oil free air pressure system or centrifugal wheel.
- AIR PRESSURE BLASTING may be carried out with a water wetted medium to obviate dust hazards. To prevent re-rusting of metal substrates, an inhibitor, which must be compatible with the surface treatment to be applied, is added to the blasting medium.
- WATER BLASTING, with the same re-rusting precaution as above.
- HAND ABRASION. Emery abrasion may be used on metals such as aluminium etc., and on glass, fibreglass laminates and for rubbing down filled areas prior to coating. Hand wire brushing can be used to remove loose rust. It will not remove tight millscale from steel since it tends to burnish the surface.
- NEEDLE DESCALING. This will remove rust millscale from steel.
- POWER GRINDING. While satisfactory on concrete, it may tend to burnish when used on steel.
- SCRUBBING with surfactant treated solvent made into a paste with the addition of a proprietary abrasive detergent. This when applied with an abrasive pad, is used to remove white rust and white rust preventative oils from galvanised surfaces.
- SCABBLING AND CHIPPING. These will not only remove laitance from concrete but will also produce exposed aggregate to improve bonding of flooring systems to concrete.
- ACID ETCHING. This is used to free concrete of laitance and is detailed under Concrete (see below).
All mechanical methods on metals must be preceeded by solvent degreasing. If no anchor pattern is produced, cleaning should be followed by further solvent washing to remove debris.
Where an anchor pattern is produced, as in abrasive blasting, the surface must be blown off with oil free, dry compressed air or vacuum cleaned.
Unless a water based compound or a moisture tolerant epoxy is to be used, surfaces must be completely dry before the compound is applied.
Since porous surfaces may be apparently dry but still contain too much moisture to allow a successful application, it is essential to test for moisture. This may be done by using:
- A suitable moisture meter with facilities not only to measure moisture at the surface but also to probe into the body of the substrate.
- The “plastic sheet” test. This test is performed on large flat areas by laying sheets of impervious plastic about one metre square in tight contact with the substrate. Do this in the late afternoon. First thing the following morning lift the sheets.
- The underside of the sheet for condensation;
- The colour of the test area in comparison to an adjacent area.
Freedom from condensate and freedom from colour change denote a ‘dry’ surface.
On very large areas, such as walls and floors, run one test per 50 m2 of area.
On non-porous surfaces, such as metals, moisture problems may be experienced if the substrate is at, or around, the DEW POINT.
The dew point is that temperature at which moisture from the atmosphere will condense. Such moisture can cause freshly cleaned metal to rust or it can be entrapped as a thin, often invisible film, between successive applications.
The dew point temperature is read off from psychometric tables or charts using an input of wet and dry bulb thermometer readings. The temperature of the surface is taken using a contact thermometer and if such temperature is within 3°C of the established dew point, application of compound must not proceed.
Some Notes on Preparation of Specific Surfaces
(It is assumed that the surface has been freed of any prior treatment)
ALUMINIUM: must be degreased, abraded, solvent re-washed and treated immediately.
FIBRE: must be freed
CEMENT: of all surface dust.
BRICK: must be free of all dust and loose particles.
CONCRETE: must be freed of all laitance, loose particles, fractured aggregate on surface, shutter release oil and curing membranes. If holes in concrete have been wet drilled they must be freed of all residues of slurry. Concrete to be coated is best treated by means of water blasting or wet abrasive blasting. Concrete floors requiring resin treatment must be clean and sound. This is best achieved by scabbling, grinding or wet blasting. As a last resort, acid etching may be used. This is carried out as follows:
- Dilute commercial concentrated hydrochloric acid (38%) with 2 volumes clean water (approximately 13% strength etch). N.B. ALWAYS ADD ACID TO WATER.
- Flood the concrete with clean water. Squeegee off excess.
- Apply the 13% solution at a rate of 2m. per litre.
- Spread evenly over the surface and leave to react.
- When effervescence ceases hose off residues, brooming with a stiff broom to loosen attached material.
- Wire brush the still wet surface to ensure all unsound material is loosened.
- Rewash to remove residues.
- If texture is not adequate repeat acid etching procedure.
- After completion allow floor to dry. Wire brush to remove weak material. Vacuum to remove all debris.
- After cleaning ensure that floor is not re-contaminated before epoxy application.
- Dryness of concrete must be proved by plastic sheet test. To prove concrete to be dust free, it may be wiped with a piece of black velvet. Velvet should remain black.
CERAMIC (UNGLAZED): The surface must be free of contamination and should be dry.
COPPER: Degrease, water wash, alcohol rinse; the surface may also be mechanically abraded to roughen.
GLASS FIBRE LAMINATES WITH EPOXY BINDER: Solvent degrease, mechanically roughen, solvent wash.
GLASS FIBRE LAMINATES WITH POLY-ESTER BINDER: Solvent degrease, abrade deeply enough to expose fibre reinforcing, solvent wash.
PLASTER: Abrade to remove all loose material followed by removal of all debris. If bonding to plaster remember that it has very poor cohesive strength.
PLASTICS: (POLYSTYRENE) Ensure cleanliness
POLYETHYLENE P.V.C. PLASTICS: Are not suitable candidates.
PLASTICS/PERSPEX: Degrease, abrade, solvent wash.
PREMIXED ASPHALT: Free of all dust and debris. Ensure that volatiles (solvent or water) have left the premix film.
PUTTY/PLASTER: Abrade well to remove loose material. Dedust. Do not bond or coat with high performance coatings. Coat only with bitumen or acrylic/PVA coatings.
RUBBER AND NEOPRENE: Degrease, abrade, rewash with solvent.
STEEL: Degrease and abrasive blast to standard SA 21/2, having a profile of 80 microns between peak and valley Treat before steel starts to rust. Any lower standard of treatment will produce correspondingly lowered performance of the treatment.
STAINLESS STEELS: Are not suitable candidates.
STONE: Must be free of contamination.
TIMBER: Oily timber such as teak must be acetone washed to free the surface of oil. For coating, timber should be as smooth as possible. For bonding, timber should be roughened by course abrasive or rasping.