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FAQ: What is Galvanizing?
-- and what if the paint won't stick?

What is hot-dip galvanizing?

Iron and steel rapidly rust and corrode. An excellent way to deter rusting for a very long time is to hot-dip galvanize the parts, that is, to dip them in molten zinc (rather like dipping strawberries in chocolate).

Why is galvanizing such a good rust-preventer?

Galvanizing is a great rust preventer for several reasons:

  • Galvanized coatings are metal (zinc), so they are much stronger and harder than paint or powder coating; they will adhere where paint may scrape off or chip off.
  • Zinc is "anodic" to iron and steel. This means it will corrode preferentially, electrochemically protecting the steel. Even if the coating is scratched through and the steel is exposed, it will be protected the same way zinc anodes protect steel-hulled ships. No organic coating, and few metal coatings, will protect iron and steel from rusting where the coating is scratched.
  • Galvanized coatings are thick. Alternatives like zinc plating (electro-galvanizing) are usually only something like one-fifth to one-tenth as thick as hot dip galvanized coatings. The life of the coating, and the part, is essentially proportional to the thickness of the zinc coating.

What are the down sides of galvanizing?

  • Because galvanizing is thick, it can't be done on standard nuts and bolts; rather the threads have to be specially cut to allow room for the galvanized coating, or the inside of nuts left uncoated. Zinc plating and most other electroplating alternatives are thin enough that nuts and bolts don't have that problem.
  • Most people don't find galvanizing to be a very decorative finish.
  • Unless special measures are taken, paint does not stick well to galvanizing.
  • Some parts can't be easily galvanized. For example, sealed tubes would be very dangerous to dip into molten zinc because water trapped inside could flash to steam and blow molten zinc out of the dip tank.
  • It is dangerous to weld galvanized parts without proper hoods and excellent ventilation that assure the fumes will not be inhaled (look up metal fume fever).

Can galvanizing be reliably painted or powder coated?

Yes, galvanizing can be painted, but it may not be easy -- especially if painting was not anticipated when the parts were galvanized.

If you are a homeowner or hobbyist trying to paint a galvanized surface, the best advice (compiled from dozens of public forum threads on this site) seems to be: clean the surface with a scrub brush and a detergent like trisodium phosphate (TSP), rinse, neutralize the detergent and acidify/etch the surface with diluted vinegar, rinse, apply epoxy or a self-etching primer made specifically for galvanized surfaces; avoid alkyd paints

If you are a manufacturer, the above will not be good enough! The best plan, used by auto manufacturers, is to employ galvanneal steel rather than galvanized steel if you intend to paint it. Galvanneal has been annealed after galvanizing in a way that causes the steel and zinc to interdiffuse so that you are painting a steel alloy rather than pure zinc.

Manufacturers may also wish to skip the chromate final dip if the parts will be painted because the brightness this dip imparts will not be required for painted parts. In any case, the galvanized or galvannealed steel must be phosphatized before beginning the painting process.

For detailed information on painting galvanized steel, manufacturers and specifiers should see "Duplex Systems: Painting Over Hot Dip Galvanized Steel", which can be downloaded as a '.pdf' file free from the AGA at www.galvanizeit.org/aga/aga-publications/download/non-member/931

This is only a very brief intro to galvanizing. Here is a partial list of related threads/ letters on the Finishing.com Hotline public forum:

  • 1126 Painting a rust-stained galvanized roof
  • 1341 Powder coating adhesion problem
  • 6590 How to paint a galvanized metal roof
  • #22099 Painting galvanized swing set
  • #31548 Painting peels off of hot-dip galvanized pipe
  • #49583 Painting a galvanized guardrail for historic preservation project
  • #55099 Getting powder coating to adhere to galvanizing -- with outstanding responses from highly experienced pros.

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