When all coats of a paint system are applied to bridge steel in an enclosed shop, the quality of the cleaning and painting is typically quite good. The steel is blast cleaned and painted in a controlled environment, minimizing or eliminating the concerns with temperature, relative humidity, dew point, and inclement weather that often plague work in the field. Access to all surfaces for cleaning, painting, and inspection is better than most field conditions, especially when the field work requires the use of lifts, rigging, and some degree of containment. The time between coats is often brief, and the potential for inter-coat contamination is greatly reduced. When all coats are applied in the shop (total shop painting), field painting is limited to painting connections and touching up shipping and erection damage, reducing the need for extensive rigging and lengthy inconvenience to the travelling public.
Given all the advantages of total shop painting with field touch up, why isn’t it done for every project? The reason isn’t related to the quality of the work produced in the shop; it’s related to the quality of the finished product together with aesthetics.
When all coats are applied in the shop, extra care is required during shipping, handling, and erection to minimize damage to the coating. While coating systems do an excellent job protecting the steel from corrosion, the coatings cannot withstand the impact of one member being dropped against another, or resist damage when secured or lifted using chains, or when banged with hammers. The resulting damage requires touch up in the field. If the damage is limited, repair is straightforward. Only localized access is needed, the damaged coating is removed to sound material using hand scrapers or power tools, the existing coating feathered, and one or more coats applied to the damaged area. However, if the damage is widespread, more extensive rigging and surface preparation are required. And depending on the nature of the damage, it may be necessary to abrasive blast clean large areas rather than attempt to repair each spot on an individual basis, greatly increasing the time, effort, and cost to complete the project in the field.
The localized areas of touch up will also be visible. It is essentially impossible to conduct localized touch up without each repair being visible, by virtue of difference in texture, gloss, and even shade of color. Brush and roller repairs of spray-applied coatings stand out, and the boarder of spray applied touch up is readily seen by virtue of a fine boarder of overspray around each patch. While the touch up will commonly fade over a few years and be less visible, the only way to avoid seeing the repair areas is to apply a full finish coat to logical break points on the structure.
Another complication often occurs when the deck is poured. While specifications require the complete removal of cement scum before it hardens, cleaning is often incomplete, leaving thin films of cement on webs and heavier deposits on flanges. The cement typically adheres so well that removing it by chipping, power tool cleaning or water jetting often removes the coating as well, or impacts the aesthetics by reducing the gloss. Repairs often require extensive surface preparation and painting to repair the damage.
In summary, while shop-painting offers distinct advantages related to the quality of the initial application, the overall integrity and aesthetics of the final system can be compromised if extensive touch up is required and cement scum is not thoroughly washed from the surface during pours. One compromise is to apply all coats in the shop except for the outside fascia, which receives only the prime or the prime and intermediate, depending on maximum recoat times. The expectation is that touch up on the underside of the bridge can be performed without concern for aesthetics and some cement on the coating can be allowed to remain. Since the fascia would already be scheduled for one or two field coats, damage to the paint during handling or when removing cement scum is less concerning since there’s no finish coat to salvage. Instead all deficiencies will be corrected during cleaning and the application of the specified one or two full coats.