GUIDELINES: PRE-TREATMENT FOR PAINTING
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Guidelines for the pre-treatment of galvanized surfaces have been drawn
up as a result of (i) a study carried out by an independent research
centre and a leading UK paint manufacturer into the performance of
commercially available pre-treatment and paint systems and the
parameters affecting their performance on hot dip galvanized coatings,
and (ii) many years experience with duplex coatings.
Although pre-treatment of galvanized components is best carried out
immediately after galvanizing, before the surface has become
contaminated in any way, this is not always practical. Pre-treatment
can be carried out later but it is vital that the surface is adequately
cleaned to remove all traces of contaminants such as oil, grease and
dirt. The cleaning operation must leave no residues on the cleaned
surface and any wet storage staining should be removed using a stiff
brush. Washing down the coating with water will help to remove soluble
salts.
There are four recognised methods of surface pre-treatment that
produce a sound substrate for paint coating:
T-Wash (or its proprietary equivalent)
Despite the fact that this preparation process has been available for
some considerable time, T-Wash is still generally considered to be the
best pre-treatment method for painting galvanized steel. T-Wash is a
modified zinc phosphate solution which contains a small amount of
copper salts. When applied, a dark grey or black discolouration of the
zinc surface will result. T-Wash must not be allowed to pool on
horizontal surfaces or this will prevent maximum paint adhesion. Any
excess should be removed by water. T-wash is most suitable for
application to new galvanizing and should not be used on weathered
galvanizing (see etch primers)
Constituents of T-wash
The constituents of T-Wash are phosphoric acid (9.0%), ethyl cellusolve (16.5%), methylated spirit (16.5%), water (57.0%) and copper carbonate(1.0%). Variations to this composition may exist and so it is wise to consult the supplier if a successful result is to be achieved.
Sufficient time must be allowed for the T-Wash to react and dry
thoroughly before the first coat of paint is applied. (Suppliers'
information will give recommended time intervals). While research has
shown that T-Washed surfaces can be left for up to 30 days before
painting and good paint adhesion can still result, it is advisable to
minimise the time between pretreatment and paint application. Any white
salt formed by the exposure of the T-Washed surface to moisture must be
removed before painting, using a stiff brush. If the T-Washed surface
has become contaminated it must be cleaned in accordance with the
suppliers' recommendations.
Etch primers
Etch primers have also been used successfully. Their major disadvantage is the absence of any visible colour change as is the case with T-Wash.
Therefore, there can never be complete confidence that all surfaces
have reacted with the primer. Etch primers are most suited to
application on older, weathered galvanizing.
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Sweep blasting
A mechanical method of pretreatment is sweep blasting using fine copper
slag, J blast or carborundum powder with a blast pressure of no greater
than 40psi (2.7 bar). This will ensure that only the minimum amount of
oxide is removed and the zinc surface is left in a slightly roughened
condition. Care should be taken when carrying out sweep blasting on
very thick galvanized coatings to avoid damage to the coating. The
optimum nozzle- to-work piece distance and angle of blasting needs to
be identified for all surfaces on the galvanized steel work if optimum
results are to be achieved. Angular iron blasting grit must not be used
under any circumstances. Sweep blasting is often used in addition to
the chemical preparation stage.
Weathering
This process only becomes fully effective after a galvanized surface
has been exposed to the atmosphere for a period of at least six months.
The surface is prepared using either abrasive pads or a stiff brush to
remove all loose adherent materials and making sure that the bright
zinc surface is not restored.
This is followed by a hot detergent wash and rinsing with fresh clean
water. The surface must be fully dry before any paint is applied.
Weathering should not be used as a method of surface preparation in
marine environments with high chloride levels.
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