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Cut Ends and Welds on Galvanized Steel

By metal gates georgetown files

Galvanized steel arrives with a protective zinc skin, and every cut, drill hole and weld breaks that skin. The bare metal left behind is where rust will begin if nothing is done. Handling these spots correctly is one of the most important parts of fabricating a galvanized gate, and it is also one of the most commonly skipped. A careful shop treats each cut as an open wound that must be closed.

What happens when galvanized steel is cut

Zinc protects steel in two ways. It forms a barrier that keeps water and air away, and it corrodes in place of the steel nearby, a process called sacrificial or galvanic protection. That second effect is why a small scratch in galvanized steel does not rust right away: the surrounding zinc keeps protecting the exposed edge for a while. A narrow cut edge on thin sheet may stay protected for years by this effect.

The effect has limits. It works across short distances, and the zinc is slowly consumed as it protects. A wide area of bare steel, such as a freshly cut tube end or a ground weld, gets only partial help from the remaining zinc. Over time the exposed metal rusts, and the rust stains and spreads. That is why any field cut should be sealed.

The special problem of welding

Welding galvanized steel is harder than welding bare steel. The heat of the arc boils the zinc away, creating white fumes that are harmful to breathe. Welders working on galvanized material need good ventilation or a respirator rated for metal fume, and many prefer to grind the zinc back from the joint before welding. The zinc also contaminates the weld pool, which can lead to porous, weaker welds if the technique is careless.

After welding, the area around the joint has lost its zinc for a width of up to an inch or more on each side. The weld itself is bare steel with a layer of dark scale on top. This region is the weakest point in the coating and needs to be rebuilt.

Cleaning the area before repair

1. Chip away weld slag with a hammer and wire brush.
2. Grind or wire brush the weld and the surrounding burned area down to bright metal.
3. Feather the edges of the surviving zinc so the repair blends into it.
4. Wipe the area clean of dust and oil with a dry cloth or a suitable solvent.
5. Apply the repair coating the same day, before fresh rust forms.

Clean metal is the foundation of a lasting repair. A coating over rust or scale peels, and the rust continues underneath it.

Choosing a repair product

Three common repair methods exist. Zinc rich paint, a liquid with a high percentage of zinc dust in a binder, is brushed or sprayed on in two or more coats. Zinc spray, sold in aerosol cans, works the same way and suits small repairs. Zinc solder sticks, melted onto the heated metal, deposit pure zinc and give the thickest repair, though they take more skill. Each has a standard behind it that sets minimum zinc content and thickness for a repair on a galvanized part.

Whatever the product, the goal is a film about as thick as the original galvanizing, built up in coats. A single thin pass leaves pinholes. Two or three passes, each allowed to dry, give a continuous layer. The repair should overlap the surviving zinc by an inch or so.

Drill holes, bolts and cut ends

Every hole drilled through galvanized steel exposes a ring of bare metal inside the hole. Where a bolt will pass through, the hole is hard to coat after assembly, so the best practice is to drill before galvanizing whenever possible. When holes must be added later, they should be cleaned and touched up before the bolt goes in. Stainless or galvanized bolts with washers prevent a mismatch of metals, and a bead of sealant under the head blocks moisture from the joint.

Tube ends are especially vulnerable. A cut end exposes the full wall thickness and, in hollow tube, an open path into the interior. Cutting a tube to length after galvanizing leaves the inside uncoated near the end. A welded cap, a plug or a generous dose of zinc rich paint brushed well inside the end closes the opening. Sharp cut edges should be eased with a file so the coating can wrap around them instead of pulling thin.

Common mistakes

• Painting over welds without removing scale, so the coating peels within months.
• Leaving cut ends bare because the surrounding zinc seems to protect them.
• Using ordinary primer where zinc rich paint is needed for sacrificial protection.
• Applying one thin coat and calling the repair finished.
• Welding without grinding the zinc back, which produces weak, porous joints.

Topcoats over repairs

If the gate will also receive a colored paint or powder finish, the repaired area should be compatible with it. Zinc rich paints accept most topcoats once dry. Powder coating over galvanized steel needs special care, because trapped gas in the zinc can bubble up during the oven bake. Shops that powder coat galvanized work often bake the part first to release the gas, or use a primer designed for the purpose.

Checking the work

A visual check tells much. A good repair is uniform in color, a dull gray that blends with the surrounding zinc, with no bare spots, runs or flaking. A finger rubbed across the area should not remove loose powder. Over the first year, watch the repaired areas for brown streaks, the early sign of rust. A small touch up at that point adds years to the life of the gate. With attention to each cut and weld, a galvanized gate can protect itself for decades.

A short field routine

For any cut, drill or weld made on a galvanized gate after it leaves the plant, a short routine keeps protection intact. Mark the area, remove burrs and scale until the metal is bright, wipe it clean and apply zinc rich paint in two or three thin coats, overlapping the surviving zinc by an inch. Let each coat dry as the label directs. Finish with a topcoat if the gate has a colored finish. The whole routine takes minutes per spot, and the cost of skipping it shows up as rust within a year or two.

Keep a small kit on hand: a wire brush, a file, zinc rich paint, a disposable brush and a rag. Gate repairs, hinge swaps and latch changes all create small areas of bare metal, and having the kit ready makes it easy to treat each one the same day.

Photographs help with record keeping. A picture of each repaired area, with a note of the date and product used, makes later inspections easier. If brown stains appear at a repair, the record shows how old it is and what was used, so the next fix can be better. Careful habits like these turn galvanized steel from a good material into a lasting one.