Learn How You Should Prep Galvanized Steel and Rusted Surfaces Prior to Painting.
By Jim Hingst @hingst_jim
The
key to any good paint job is good surface preparation. Preparing to paint steel
surfaces requires special care.
Before
sanding of the substrate, you must clean any oil or grease from the surface
using a wax and grease remover and a final wipe down with alcohol. Most paint companies formulate their own
solvent cleaner for removing grease and oil.
Trying
to abrade the contaminant off of the substrate without cleaning the surface,
only makes a bad situation worse. The reason is that by sanding a contaminated surface
you only drive the contaminant into the old paint or metal pores.
Treating Rusted Surfaces.
Solvent
cleaning will do nothing to remove rust from steel. Rust is the common name for
iron oxide. It is like cancer. Once it starts, you need to
kill it completely or it will continue to penetrate the metal causing more
rust.
If
a surface is rusted, the only way to eliminate it is to sand down to bare metal
or sandblast the surface. The problem with trying to grind away rust is that
the abrasive usually doesn’t get into the tiny pores of the metal, so it leaves
some of the rust behind.
To
dissolve the residual rust or to convert it into black oxide, automotive paint
shops use an acid metal wash, which is a mixture of an acid, such as phosphoric
acid, and glycol ether. The acid mixture not only cleans the metal, but also
conditions it by chemically reacting with the metal to form a thin layer of
zinc phosphate.
This
protective layer prevents moisture from reacting with the steel or iron. Two of
the commercial solutions used by automotive body shops that are formulated to etch
rust are DuPont Quick-Prep and Sherwin-Williams Metal Prep.
After
coating the bare metal surface with the metal prep, you should be good to
go. However, many painters will take the
additional steps of wiping the surface down with solvent cleaners.
When working
with any type of acid, read the manufacturer’s Safety Data Sheet (SDS),
technical bulletins and other related literature, before using the product. Always wear rubber gloves, safety goggles and
other required personal protection equipment (PPE), when using acids.
Galvanized Steel.
Galvanized
steel is a special case. It has a special protective coating of zinc that
prevents rust. Here’s how this special coating works. Instead of the steel oxidizing and rusting,
the zinc oxides instead. As the metal
oxidizes, what is called “white rust” appears. Failure to remove this white
rust will compromise the adhesion of the paint resulting in peeling. To get rid
of the white rust you can use a wire brush or an abrasive pad.
After
the zinc corrodes, the steel will inevitably rust. Painting the galvanized metal provides
additional protection and improves its appearance. Before painting galvanized
steel it must be primed.
New
galvanized steel is one of the most difficult surfaces to paint. While the
paint may initially stick to the surface, it eventually fails. It is just a
matter of time.
The
oily coating on new metal usually prevents good paint adhesion. This oily layer
must be removed using a special solvent cleaner specified by the paint
manufacturer.
Solvent
cleaning alone is not sufficient. New galvanized metal is still a slick
surface. To give the surface some tooth,
you must then treat the surface using a cold phosphating pretreatment.
After
treating the surface, it is ready for primer and paint. Standard paints used in
the sign industry generally will not work.
You must use primers and paints formulated specifically for galvanized
steel.
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About Jim Hingst: After fourteen years as Business Development Manager at RTape, Jim Hingst retired. He was involved in many facets of the company’s business, including marketing, sales, product development and technical service.
Hingst began his career 42 years ago in the graphic arts field creating and producing advertising and promotional materials for a large test equipment manufacturer. Working for offset printers, large format screen printers, vinyl film manufacturers, and application tape companies, his experience included estimating, production planning, purchasing and production art, as well as sales and marketing. In his capacity as a salesman, Hingst was recognized with numerous sales achievement awards.
Drawing on his experience in production and as graphics installation subcontractor, Hingst provided the industry with practical advice, publishing more than 150 articles for publications, such as Signs Canada, SignCraft, Signs of the Times, Screen Printing, Sign and Digital Graphics and Sign Builder Illustrated. He also posted more than 325 stories on his blog (hingstssignpost.blogspot.com). In 2007 Hingst’s book, Vinyl Sign Techniques, was published. Vinyl Sign Techniques is available at sign supply distributors and at Amazon.
© 2016 Jim Hingst
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