Field galvanizing repair per ASTM A780

ASTM A780 governs field repair of damaged or uncoated areas on hot-dip galvanized coatings with three methods: zinc-rich paint, zinc-based solder and zinc metallizing. In every case, the area is cleaned to clean metal, the repair overlaps the sound zinc around it, and thickness is measured against the requirement of the original galvanizing standard.

Hot-dip galvanized steel rarely reaches the end of erection intact. Field welds, cuts, drilling, slings and impacts create areas where the steel is bare. Without touch-up, these areas become the first rust spots on the structure. This guide is for project managers, superintendents and maintenance managers in Quebec and Ontario who need galvanizing repaired on site, alongside our comparison of cold galvanizing vs hot-dip.

What does ASTM A780 cover?

ASTM A780 covers repair of damaged or uncoated areas on hot-dip galvanized coatings, whether the damage comes from the plant, transport, erection or maintenance. It describes the permitted materials, preparation, application and thickness of repaired areas.

The standard works together with the original galvanizing standard: ASTM A123 for structural products, ASTM A153 for hardware and ASTM A767 for reinforcing bar. These standards also state when repair is permitted at the galvanizer and beyond what extent the part should be regalvanized instead. The project specification states which applies and which touch-up method is selected.

Repair is justified because zinc protects steel galvanically: it corrodes in its place, as our galvanic series table shows. A zinc touch-up restores that protection, which ordinary paint does not.

Which touch-up methods does the standard allow?

The standard allows three methods, each described in an annex: zinc-based solder, paint containing zinc dust and sprayed zinc. They differ in equipment, preparation and the kind of job they suit.

Zinc-rich paint is the most common field method for routine touch-up. See the zinc-rich primer entry. Metallizing becomes attractive when areas are large or when the durability of the repair has to approach that of galvanizing; our thermal spray metallizing service sets out the conditions.

How do you prepare the area to be repaired?

You prepare the area by removing anything that prevents adhesion or contact with the steel: oil, grease, weld slag, rust and flaking zinc. The repair has to sit on clean steel and overlap the sound galvanizing around it.

• Degreasing of the area and its surroundings, per SSPC-SP 1.
• Weld residue removal: slag, spatter and oxides, by grinding or brushing.
• Cleaning to clean metal, by spot blasting or power tools, to the grade given in the method's annex and in the specification. Our article on power tool cleaning SSPC-SP 3, SP 11 and SP 15 describes the power tool grades.
• Roughening the adjacent zinc over a band around the area, so the touch-up bonds to it.
• Dry surface above dew point at the time of application.

For solder, the area is also heated to the temperature given by the standard and the alloy manufacturer, without overheating the adjacent coating. For metallizing, abrasive blasting with a profile is required.

What thickness should you target and how is it measured?

Target thickness is derived from the original galvanizing standard: ASTM A780 ties it to the thickness required by ASTM A123, A153 or A767, with specific rules for zinc-rich paints. The specification carries the applicable value; failing that, it is taken directly from those standards.

• Measurement with a magnetic or electromagnetic gauge, per ASTM E376 for zinc and SSPC-PA 2 for paints, at several points spread over the area.
• Zinc-rich paint: thickness is built in successive coats, respecting the drying time between coats in the data sheet.
• Recording of readings in a dry film thickness log, with the location of each repaired area.

A touch-up that is too thin is consumed quickly; a paint coat that is too thick can crack or dry poorly. In both cases, measuring avoids guesswork.

Which method fits which case?

You choose based on the size and number of areas, access, whether heating or blasting is possible, and the treatment planned afterward. The table above gives the trends; a few cases come up often.

• Erection welds and small cuts: zinc-rich paint, or solder if the crew is equipped and the area is accessible.
• Bolts and connections: zinc-rich paint on damaged areas, in line with our article on bolt and connection corrosion protection.
• Large damaged areas or missing galvanizing: metallizing, or regalvanizing the part if it can be removed.
• Structure to be painted: zinc-rich paint compatible with the finish system.

Which mistakes make a touch-up ineffective?

Most touch-ups that rust quickly fail because of preparation or thickness, not because of the method chosen. The same mistakes come up from one job to the next.

• Painting over weld slag. Slag is alkaline and friable; the touch-up comes off with it.
• Touch-up limited to the bare spot. Without overlap onto sound zinc, corrosion starts at the junction.
• A single coat of zinc-rich paint. The required thickness is almost never reached in one coat.
• Wrong product. A galvanized-look paint without enough zinc does not protect steel galvanically.
• Wet or cold surface. Condensation and low temperature compromise adhesion and drying.
• Missed areas. Hole bores, the underside of base plates and cut edges are often skipped.
• No record. Without a list of repaired areas and readings, nobody can confirm later that every spot was treated, and the next inspection starts from zero.

When a touch-up has already failed, our article on coating failure modes helps find the cause before redoing it. A quick look at the failed area, the product used and the conditions at application usually points to the fix, and avoids repeating the same error on the next structure.

Should the touch-up be painted over?

It is not mandatory, but it is common when the structure receives a duplex system or is exposed to an aggressive environment. The touch-up then becomes part of the paint system preparation.

Compatibility between the touch-up and the finish has to be checked: a zinc-rich paint must be declared compatible with the next coat, and the surrounding galvanizing is prepared per SSPC-SP 16. Our article on the duplex system on galvanized steel details that preparation, and our protective coatings page presents complete systems.

How do you get a quote?

You get an accurate quote by describing the number and approximate size of the areas, their access and the original galvanizing standard. Our crews and qualified subcontractors carry out these repairs from LaSalle, including Ontario, Montreal and the industrial regions of Quebec.

Send photos, drawings or a list of areas and the specification requirements through the request a quote form; our cold galvanizing page presents all the options. Pricing is on consultation, based on method, access and volume, and we reply within 24 business hours.

Frequently asked questions

Is silver paint enough to touch up galvanizing?

No. An aluminum-look or silver paint without enough zinc dust does not protect steel galvanically. ASTM A780 requires a zinc-rich paint that meets its requirements.

Can touch-up be done in cold weather?

Yes, if the surface is dry, above dew point and within the temperature range of the product data sheet. For metallizing, blasting conditions also apply.

Do holes drilled after galvanizing need touch-up?

Yes. Hole edges and bores expose the steel. They are cleaned and touched up with the same method as other areas, before bolts are installed where possible.

Who checks the touch-up?

The owner's or general contractor's inspector, as set out in the specification. They check preparation, thickness and appearance, based on the recorded readings.