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Preparing
Galvanized Steel for Painting
Summary
This details a recommended method for prepping
sheet galvanized used for ventilation ductwork,
roof decking and many other applications. New
galvanized steel of this type usually has a clear,
glossy, "passivation" film which will
cause the loss of coating adhesion if it is not
removed.
The passivation film is applied
by the galvanizer to prevent the zinc from reacting
with water and carbon dioxide during transportation
and storage.
Unprotected zinc forms white,
powdery deposits of zinc oxides, hydroxides and
carbonates, that are commonly referred to as "white
rust", "zinc salts" or "zinc
soaps." If white deposits on an unpassivated
zinc surface are not removed prior to coating,
coating failure usually occurs.
The glossy passivation film
is often misidentified as a wax or an oil. It
is neither and cannot be removed by solvents such
as xylene. The film is a thin coating of an alkaline
liquid, similar to the "cure & seal"
used on concrete, which can be removed with Great
Lakes Laboratories Clean 'n Etch, an odorless,
acid-based detergent.
Equipment
- Rubber Gloves, Dust Mask
- Safety goggles or face shield
- 2.5 - 3 gallon, all plastic,
hand-pump, tank-type, garden sprayer
- Water for rinsing can be
applied with an airless sprayer, water hose
or garden sprayer.
Procedure
- Establish the concentration
of Clean 'n Etch required to achieve the desired
results. See section titled "Determining
Concentration".
- Mix Clean 'n Etch in required
strength using hot water, if available.
- Adjust the spray nozzle for
a medium spray to reduce aerosol effect.
- Work in sections to insure
that the surface is rinsed before the solution
begins to dry.
- Wet the surface with the
solution and allow 5-7 minutes contact time.
- Rinse treated section thoroughly
with water.
- Air dry or force dry.
Usage Tips
- Each +18°F increase in
the temperature of the solution will double
the rate of reaction. Using hot water will reduce
treatment time and may reduce concentration
required.
- Use oil-free compressed air
or an electric leaf blower to reduce drying
time.
Tests for Paint Readiness
- Copper Sulfate Test (Best
Method)
Apply 1 drop of a 10% copper sulfate solution
to the treated/rinsed/dried surface. If a black
spot develops within 5 seconds of contact, the
surface is ready for painting.
- Water Break Test
Spray water on the treated/rinsed/dried surface.
If the water beads or breaks, the surface is
not ready for paint. If the water sheets over
the surface, it is a good signal that the passivation
film and any other oily soil has been removed.
Determining Concentration
- Mix 1 oz. Clean 'n Etch with
1 oz. water
- Apply to a test area that
represents the surface to be treated.
- Wait 5-7 minutes. Rinse and
dry.
- Test the surface for paint
readiness.
- If the surface is "paint
ready", add 1 oz. of water to the solution
and repeat steps A-D, add 1 oz. of water each
time the test shows that the surface is "paint
ready". When the solution becomes too weak
to provide the desired result, use the previous,
stronger dilution.
Troubleshooting
White deposits on the treated surface may occur
if the concentration is too strong, the contact
time is too long or the solution is not rinsed
before drying begins.
These deposits are zinc phosphate
resulting from the chemical reaction of Clean
'n Etch with zinc on the surface
and usually do not appear if the surface is rinsed
before drying.
A light amount of zinc phosphate
will not affect coating adhesion, however, it
is good practice to brush off as much of the white
powder as possible prior to coating application.
For Technical Assistance
Call:
1-800-888-1105
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