Natural Weathering & Zinc Repairs
What Touch-up & Repair Materials Weather Naturally with the HDG Coating?
Although galvanized coatings are highly abrasion resistant, coating damage may occur once the galvanized steel leaves the galvanizer’s facility due to extremely rough handling, installation techniques, welding after galvanizing, or in-service conditions. These exposed areas of steel require repair of the hot-dip galvanized coating to protect the steel and ensure the integrity of the coating.
ASTM A780, Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings, discusses the use of three different materials for repairing these damaged or uncoated areas: zinc-rich paint, zinc-based solder, or zinc-spray metallizing. The use of touch-up materials can alter the initial appearance of the galvanizing finish as they are not usually an exact color match. Although differences in appearance from the use of touchup materials is acceptable and not grounds for rejection, selection of repair material can be considered when aesthetics are a primary concern.
In terms of appearance, zinc-based solders (Figure 1) and zinc-spray metallizing (Figure 2) will more closely match the appearance of hot-dip galvanized steel and will weather similar to the base galvanizing. However there are limitations to applying both materials which must be considered in addition to cost.
Because the galvanized steel surface will weather and become a matte gray color over time (6 months to 2 years in the environment), zinc-rich paints can eventually provide a uniform aesthetic if a matte gray or weathered color repair paint is utilized. Although the appearance may at first be a mixture of gray paint and shiny hot-dip galvanizing (Figure 3), all hot-dip galvanized coatings will weather to a matte gray and more closely match the repair paint color over time. Touch-up performed using bright/shiny zinc-rich paint (Figure 4) will not weather in color similar to the surrounding coating, and will therefore result in permanent mixed appearance once the base galvanizing weathers. If required for aesthetic purposes, top-coating of the touched-up area with aluminum or bright/shiny spray-applied paint products can be used to color-match newly galvanized steel.
To assist in selection of repair materials, AGA has an updated list of HDG repair material suppliers along with the primary repair products available. Some of the AGA zinc-rich paint suppliers offer shiny repair paint, matte/weathered color paint, or both.
Touch-Up & Repair Material Suppliers 2019
Evaluation of HDG Repair Materials
© 2023 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.
Central Texas Iron Works
Touch up and Repair ? I have been around the structural steel fabrication industry for over 40 years and have invested many hours visiting galvanizers and Fabricators in many cities, states. What I have seen for sometime now is how much more the grinder is being used during the finishing process. An I beam or girder beam has 12 surfaces and when submerged in a zinc bath then brought out slowly and tilted the zinc material is still bonding and some of the zinc will run towards an edge and will continue to run until it hardens enough and forms ice cycles or runs and build up in corners. But at the edges grinders are seem to be used more to help speed up the finishing process. Many of my visits to a galvanizer is to look at the first 3 loads of a project we have and get a good feeling for the coating. During these visits and walking thru the laydown yard where the finish product is staged by loads and ready to be picked up and loaded on to a trailer I notice several edges with rust. And its where grinders have been used. Now I know it takes a little while for it to appear but when moisture is in the air it doesn't take that long. I ask the same question every time, why don't you have these touched up before I get here. What is the proper touch up for this?
Hello, Some galvanizers use metal files or a combination of grinder/metal files to avoid damaging the coating. If for some reason too much zinc is removed (or down to bare metal), it should be repaired by the galvanizer. The methods for touch-up of these areas is exactly the same as touch up and repair of bare spots at the galvanizing plant or the field (zinc-rich paint, zinc-solder, or zinc metallizing).