There are a few tricks to clean concrete before sealing.
Even new concrete is unsuitable to seal because the
surface is too
tight while old concrete is too dirty.
If you
coat over these without first using a
concrete grinder or
other methods below you will only achieve partial adhesion
of the sealer in small
areas and the majority of the floor will suffer from
peeling.
Peeling is a layer of coating material that has
never adhered fully to the surface and as it dries and
hardens it de-laminates away cleanly leaving a bubble of air
underneath. Thin, clear sealers will turn white where the
adhesion has given way.
To clean concrete before sealing the surface needs to be
cleaned of any contaminants and be porous to allow the
sealer to key into the concrete.
Driveways & pavement
Concrete that has been weathered for years may already
have the shiny top layer removed naturally and all it needs
is a good clean with a water jet to prepare it. As a test
you should hand scrub a few small areas the size of your
hand with laundry powder detergent, water and a stiff brush,
then rinse it and allow it to dry.
When the areas have thoroughly dried drip some clean
water on them to see how porous the concrete is and how
readily the water is absorbed because this is the best
indication of how easily the paint will be absorbed. Choose
an area that is subjected to high traffic, an area that is
sheltered and another area to the side that has never been
worn and compare each one.
Suppose all areas absorb water immediately, you could
water jet clean the entire area and then paint when it
is dry making sure that the first coat is thinned 10%.
Water jet cleaning of concrete is quick, easy and an excellent way to
prepare worn concrete because it removes dirt, algae and
dust from the small pores of the surface. As long as it is
rinsed properly without leaving residue to settle back into
the pores the concrete is ready to coat immediately it looks
dry.
There are two main concerns with preparation of worn
concrete - one is sheltered sections that still have a tight
surface which will have to be opened and the second is oil that has soaked into the
concrete.
Oils stains
Oil will eventually soak right through concrete over time
aided by the sun to lower its resistance and the opening and
closing of the concrete when it heats up and cools down. The
good news is that you only have to remove it from just below
the surface to achieve good adhesion and once the paint has
dried it will stop the oil from returning to the surface
unless the oil is very wet all the way through. If the oil
is very wet you will need to extract most of it first with a
solvent and an absorbent that can be found in any hardware.
To extract oil from concrete you need to wipe the area
with a rag after applying Turpentine or a similar solvent to
remove most of the surface material. Then cover the area
with an absorbent product such as is used in cat baskets to
absorb waste and apply more Turpentine to wet it. Cover with
plastic sheet and allow it to work overnight. The oils will
be diluted and as the solvent evaporates it will be trapped
in the absorbent. Very bad areas may need two treatments.
Oil stains including those treated with the solvent can
often be removed with simple laundry detergent powder and
water, however the older stains will contain dirt which may
be impossible to remove completely. Wet the surface and
sprinkle with the detergent, rub it in with a broom making a
wet paste and leave it for 30 minutes if possible wetting is
slightly during this time if it appears to be drying out.
When rinsed the stain should have vanished or be mostly gone,
but it will return within 12 hrs in hot weather as the oil
comes back up to the surface again.
If this happens it can
be cleaned again, dried and painted as soon as possible to
allow the paint to dry before the stain reappears. Using a
wet vacuum will dry the areas quickly without wetting the
rest of the concrete.
Three ways to open the concrete surface
After completing your test areas in different parts of
the concrete if you find there are areas where the polish is
still on the concrete where the water test does not absorb
into the surface it will need to be opened with either shot
blasting, acid etching or a
concrete grinder. These three
processes will remove the shiny area on top of the concrete
and open up the porous concrete underneath. When this has
been achieved your paint will adhere strongly until it
finally wears out through constant use.
How to clean concrete by:
Acid etching
Caution must be exercised when using acid to etch
concrete due to the possible health problems and the damage
it may do to plants, clothing or nearby equipment. This
article will describe the basic process, but you must follow
the safety instructions given to you by the seller of the
product or as indicated on the label.
Hydrochloric acid which is mainly sold for brick cleaning
will attack the lime in the concrete and dissolve the bond.
When it is used to clean a concrete floor, driveway or pavement it
is quickly neutralized or mostly neutralized on contact or
when the fizzing has stopped so it only acts on the surface.
Usually a mix of 10 parts of water to one part of acid will
be strong enough to remove the polish from the surface.
If
you wear correct protective clothing, gloves and eye
protection and take care not to splash, the best way to
apply the acid is with a long handled paint roller with a
deep pile cover. Apply it liberally and slowly enough to
prevent a spray.
After applying the acid/water mix and before it dries
thoroughly water jet clean the concrete by blasting it with
3,000 psi close to the concrete, rinsing is not good enough
to remove the surface particles and a rotary jet (like a
lawnmower) is the best way. When the concrete is dry apply
the first coat of paint.
Shot blasting
This is better left to the experts who know what power of
machine to use, what size shot and how fast each pass should
blast the surface. These machines throw small steel balls at
the concrete within a sealed chamber by passing the shot
through a fast-spinning paddle wheel. The shot hits the
surface, breaks it open and then bounces back up the return
chamber to be recycled while a vacuum system separates and
removes the dust and grit. Shot blasting can open the
surface more than intended and leave deeper grooves where
the blasting overlaps so it is not ideal for home
applications.
Concrete grinder
These devices come in all shapes and power sizes as well
as wheel configurations, from small hand held angle grinders
with a dust extraction shroud and diamond grinding wheel to
large walk-behind machines that have multiple wheels
underneath. For driveways that need only the edges opened at
the surface an angle grinder set up as a
concrete grinder is
ideal. It will grind dry and the dust collector will remove
all the dust as you work. For larger areas more than the
edges a sit down grinder or stand up grinder using an angle
grinder as the motor source will be faster and easier. There
is a dust extraction shroud that even gets into the corners
and most have edging features to grind against walls.
They work by using a spinning wheel in contact with the
concrete that has diamond segments welded to the wheel. As
these diamonds come in contact with the concrete they wear
away and grind off the top surface. The dust extraction
shroud contains all the dust and the dust collector removes
it without airborne particles.
Painting floors
Once your surface is open and clean apply the first coat
thinned 10% to soak into the surface as deeply as possible.
If your preparation is good it will look dull and lifeless
when it has dried. Depending on the coating, apply another
coat once the first has fully dried which will give you a
glossy, uniform finish. Apply a third coat if a greater
thickness is required for better wear.
