What is a primer?
A primer primes the surface to receive paint. It provides better adhesion to the substrate for the final finish paint than if the finish paint were used alone. As primers are usually used as an undercoat, its aesthetic properties can be sacrificed to provide other benefits such as binding, filling and anti-corrosion, as in the case of metals.
The main objectives of any metal primer are :
1. To provide better adhesion to the top coat paint.
2. To provide protection to the metal surface.
Most metal primers are designed to work with ferrous metal substrates. Metal Primers for aluminum are also available.
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| Corrosion can be prevented by using Metal Primers |
1. Passive Barrier Protection
Protect through the barrier properties of resin or pigment having low water transmission. (i.e. epoxies, chlorinated rubbers)
2. Active Protection
Inorganic zinc inhibitive pigments, such as zinc phosphate, offer active anti-corrosive protection to the steel substrate.
3. Sacrificial Protection
Zinc metal in direct contact with the steel substrate offer protect through the preferential oxidation of zinc metal. Examples of sacrificial protection include inorganic zinc silicates, organic zinc-rich primers, metal-sprayed zinc, hot dip galvanising and electroplating.
Varieties of Metal Primer
Red-Oxide in a Hydrocarbon Matrix
The most common variety of metal primer is Red-oxide metal primer. Its main components are Red Iron oxide (Fe2O3) in an alkyd (hydrocarbon) binder matrix. When the iron oxide particles, which are actually small rust particles, are suspended in a hydrocarbon matrix, there will be no interaction between the oxygen in the air and the underlying metal. The iron oxide particles are inert, since they have already reacted with the oxygen in the atmosphere. Red-oxide primers are not usually suitable for galvanized or nonferrous metals like aluminum, copper or brass.
Red-oxide Epoxy primers are also available, where the Iron oxide is suspended in an Epoxy matrix. This combination give better adhesion, higher film thickness and lower water transmission than alkyd based primers, thus has better anti-corrosive properties.
Zinc-Chromate in a Hydrocarbon Matrix
Zinc chromate in a hydrocarbon matrix is the primer of choice for aluminum surfaces. Although chromium is a very expensive metal, it is very inexpensive as zinc chromate, since this is a natural form of both Zinc and Chromium metals. Zinc chromate will not react with oxygen. The zinc chromate bonds quite readily to the aluminum, so this is a highly effective primer.
Zinc-Chromate primers can also be used on ferrous metal surfaces.
Due to the presence of chromium (a heavy metal) in the primer, it is deemed as unfriendly to the environment.
Zinc-Phosphate in a Hydrocarbon Matrix
Zinc phosphate in an alkyd matrix is a very good primer for ferrous metals. Zinc phosphate (Zn3(PO4)2) is only slightly soluble in water. It hydrolyzes in water to produce zinc ions (Zn (2+)) and phosphate ions (PO4 (3-)). The phosphate ions act as anodic inhibitors by phosphating the steel and rendering it passive. The zinc ions act as cathodic inhibitors.
The Zinc phosphate pigment thus provides active protection to the ferrous metal surface.
Zinc Phosphate in an epoxy matrix is an excellent metal primer and can be used in extremely corrosive environments.
Surface Preparation
The biggest mistake is to apply primer over a
surface with water, dirt, oil, or grease on it. The primer will adhere to the
water, dirt, oil, or grease, and it will not adhere to the underlying metal.
Use a degreasing solution to remove
these materials and allow the surface to dry. Applying primer on top of water, dirt, oil, or grease is a complete waste of
time and money.
Rusted surfaces should be treated with
rust remover and scrubbed down with a wire brush or abrasive paper back
to bare metal.
To completely remove surface rust, Sand-blasting is recommended. Even sand-blasting suffers
from a phenomenon known as flash corrosion. As soon as the abrasive exposes the
iron particles, they react with the oxygen particles in the air stream used to
transport the blasting media. The iron turns from a silver color to a gray color
within milliseconds of exposure.
An iron oxide film on top of the surface to be painted is not completely unacceptable, as long as the iron oxide is firmly bonded to
the underlying metal surface. It is not a bad engineering practice to apply the
primer over the light surface oxidation.
Metal Primers must be applied
as soon as the treated surface is thoroughly dry. If the treated
surface is affected by rain or left overnight and exposed to dew, the
surface will have to be re-cleaned before applying the primer.
A clean surface ensures
that the paint, applied after the primer, adheres properly.
Application
. After satisfactory surface perparation, thin the primer to the given consistency using the recommended thinner. Usually 10-25% of thinning is recommended. In the case of epoxy primers, the base and hardner are mixed in the given proportion before thinning.
. Brush, roll or spray the primer onto the surface. Special care must be taken to cover welds and joints. Also junctions between different metals must be coated thoroughly as these can be hotspots for corrosion.
. Allow the primer to completely dry. Usually overnight is recommended.
. Time to apply the finish coat.
Precautions
A lot of paint jobs are ruined because the
painter is in too big a hurry and the primer coat doesn't dry completely. If
this happens, the solvents in the primer are trapped underneath the finish coat.
This ruins both coats of paint, since the paint was not designed to have
un-evaporated solvents in it. Paint that is suffering from vapor entrapment
never gets very hard. If paint can be scraped off with a fingernail a year after
application, it is probably suffering from vapor entrapment.
If a primer coat is applied during inclement
weather, it is highly prone to failure. The humidity can be 100% before, during,
and after rain. 100% humidity means that every surface is a condensing surface.
The film of water is not visible, but it can usually be felt. To paint over this
film totally defeats the effectiveness of the primer. The water creates voids n
the primer, so it is no longer impermeable to oxygen or water.
Once the primer has dried completely, it is
time to proceed to the finish coat.
Commercially available Primers
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| Asian Paints SmartCare Yellow Primer |
. Asian Paints SmartCare High Performance
Zinc-Phosphate Yellow Primer
. Zinc-Phosphate in a Hydrocarbon Matrix
. Thinner - Mineral turpentine, Enamel Thinner (8-10%)
. Coverage - 80-100sqft per litre (1 coat).
. Product Rating (4.5/5)
. Asian Paints SmartCare High Performance Zinc-Phosphate Red-Oxide Primer
. Zinc-Phosphate and Red-Oxide in a Hydrocarbon Matrix
. Thinner - Mineral turpentine, Enamel Thinner (8-10%)
. Coverage - 80-100sqft per litre (1 coat).
. Product Rating (5/5)
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| Asian Paints Red-oxide Metal Primer |
. Asian Paints Decoprime Red Oxide Metal Primer
. Red-oxide primer in an Alkyd Matrix.
. Thinner - Mineral turpentine, Enamel Thinner
. Coverage - 80-100sqft per litre (1 coat).
. Product Rating (3/5)
. MRF Xuper Yellow Ptimer
. Zinc-Chromate in an Alkyd Matrix.
. Thinner - Mineral turpentine, Enamel Thinner
. Coverage - 80-100sqft per litre (1 coat).
. Product Rating (3.5/5)Available in store now, at
Shree Venkateshwara Traders
712, Modi Hospital Road,
West of Chord Road, Mahalaxmipuram
Bangalore 560086
9036092531 - Namit Bhulani
080 - 23592523 - Landline
venkateshwara.traders@gmail.com
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