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Concrete is
the complex reaction product of aggregates, silica sand, and calcium
silicates present in cement. Lime is produced during the setting reaction
which slowly, further reacts with the silica sand to form more cement.
This lime production accounts for the high initial alkalinity of concrete
and is influenced by the amount of water added, type of aggregate, and
additives used. This high alkalinity can saponify (lime burn) many alkyds
and PVA's. It is recommended that concrete is left for twenty eight
days before painting with alkali-resistant paints (e.g. 100% acrylics),
three months for PVA's and one year for alkyds. Excess lime can migrate
to the surface as laitance or efflorescence. The cement matrix is slowly
eroded by acids normally present in rain, so that old concrete has a
weak, unbound layer of sand on the surface.
The above holds true for all concrete products although the different
forms do have some individual differences:
- Poured concrete - More likely to have efflorescence and
laitance due to higher slump mixes. Strong likelihood of form oils
and/or curing agents being present.
- Concrete masonry - The very low slump mixtures used in this
area reduce alkalinity but increase porosity. Mortars used for jointing
and reinforcing are generally highly alkaline.
- Cement renders (including stucco, rough cast and EIFS) - Can
have weak surface layers due to water loss from relatively thin layers
leading to poor curing. Many renders have added lime leading to very
high alkalinity.
- Fibre reinforced cement - Low incidence of alkalinity and
efflorescence in flat sheet; greater damage in moulded sheet. Some
possibility of mould release oil contamination.
- Old fibre reinforced cement - (Prior to 1982) may contain
asbestos. Local Health Authorities must be contacted for advice on
preparation work.
- Glass reinforced cement (GRC) - These very dense and glossy
surfaces can also be contaminated with form release oils and defoamers.
- Brick - A very stable material and generally only requires
cleaning prior to painting. Old bricks which have been damaged by
constant exposure to frosts may have a weakened surface and will require
a penetrative primer/sealer. The mortar between bricks will also weaken
considerably with ageing.
We recommend acrylic paints because of their inherent durability and
alkali resistance. The glossier finishes are generally tougher and more
easily cleaned but highlight surface imperfections to a greater
degree. Textured Coatings help disguise surface imperfections. If a
textured coating is required then a specialised painter is required
- contact Resene.
Surface preparation:
New concrete, concrete masonry and fibre reinforced cement:
- Remove powdery layers and efflorescence:
Remove any powdery layers, laitance or efflorescence by vigorous wire
brushing or preferably waterblasting.
- Degrease and clean surfaces:
Thoroughly degrease by scrubbing or brushing down with detergent to
remove all dirt, dust, grease, chalk, cobwebs and other contaminants.
Rinse clean with copious amounts of clean water. Repeat process if
necessary.
- Test for moisture:
Tape a 0.1mm clear plastic sheet (45 x 45cm) to the concrete surface.
Taping must provide an airtight seal between the concrete and plastic
sheet. Check the underside of the sheet after 24 hours for signs of
condensation.
Darkening of the concrete covered by the plastic sheet compared to
surrounding areas also indicates presence of moisture. Allow an additional
drying/curing time of fourteen days and repeat test.
If moisture is detected on second test, the surface cannot be successfully
coated.
Old concrete, concrete masonry, fibre reinforced cement, new and old
cement renders:
In order to assess the condition of these surfaces prior to painting,
scratch the surface with a penknife blade - if the surface can be scratched
off as a powder the whole area must be thoroughly cleaned.
- Remove powdery layers and efflorescence:
Remove any powdery layers, laitance or efflorescence by vigorous wire
brushing or preferably waterblasting.
- Special attention needed:
Fibre reinforced cement may contain asbestos. The removal and/or
disposal of asbestos containing materials (or just the cleaning down
of them) requires special permission and a licence. Contact your local
Council offices to determine your obligations under their bylaws.
Free asbestos is likely to be released during surface preparation
and therefore suitable dust masks or respirators must be worn by all
operators involved in surface preparation and disposal of waste. Special
care must be taken to contain all debris and this should be kept wet
and disposed of in a land fill. This material must not be left to
dry out. All washings must be collected and professional tradespeople
involved.
- Degrease and clean surfaces:
Thoroughly degrease by scrubbing or brushing down with detergent to
remove all dirt, dust, grease, chalk, cobwebs and other contaminants.
Rinse clean with copious amounts of clean water. Repeat process if
necessary.
- Remove all moss and mould:
Thoroughly clean down to remove all loosely adhered material. Treat
areas of moss or mould infestation with Resene
Moss & Mould Killer correctly diluted with clean water.
Leave for up to forty eight hours to achieve full kill. For heavy
infestations further applications may be needed. Wash thoroughly with
clean water to remove residues.
- Waterblast 3000 psi:
This may not be necessary, but water blasting will ensure an excellent
surface for painting. Carefully waterblast at 3000 psi to remove all
dirt, chalk, moss and mould residue and any other contaminants. Allow
the surface to dry out for at least twenty four hours.
First coat:
There are a number of important conditions which need to be considered:
- For areas which are at risk of mould (i.e. damp areas that receive
little sunlight) coat with Resene
Quick Dry Acrylic Primer Undercoat. Quick Dry contains fungicides
which discourages mould growth.
- If the cementitious surface has a powdery feel to it then Resene
Sureseal must be used. Resene Sureseal is an oil-based, alkali-resistant,
penetrating sealer. It has the ability to penetrate and bond old decaying
surfaces and make good for painting.
- For smooth/glossy surfaces use Resene
Smooth Surface Sealer - developed for use over surfaces usually
considered as being difficult to coat.
- If the surfaces are in sound condition, then proceed to the next
section without applying this primer/undercoat.
Topcoats:
Once the surfaces have been properly prepared and/or
primed/undercoated apply either Resene
Hi-Glo (gloss), Resene
Sonyx 101 (semi-gloss) or Resene
Lumbersider (satin), depending on your gloss level preference. Click
here for further information on gloss levels.
Follow label instructions at all times.
Cleaning up:
Cleaning up waterborne paints is different to solventborne paints -
click here
for detailed instructions on clean up.
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