There seems to be a trend for bridge owners to believe that applying all three coats of paint in the shop is more cost effective and will provide for a better coatings job on new steel. This paper will discuss the differences between shop coating and field painting and the pros and cons of shop applied coatings versus field applied coatings on new steel.
This paper will also discuss various cost differences between shop and field painting as it pertains to new steel and we will cover two case histories of Maryland State Highway Administration jobs which had all three coats of paint applied in the shop.
For our discussion today we will be talking about the most widely used coating system coming out of the shop for bridge coatings, which is an inorganic zinc rich primer followed by an epoxy intermediate and polyurethane topcoat.
There are three combinations of shop and field painting:
1. Shop/Shop/Shop (with field touch-up)
2. Shop/Shop/Field
3. Shop/Field/Field
• All three coats of the specified system are applied in the shop.
• The only field work is the repair of the damaged coating.
• The primer and intermediate coats are applied in the shop.
• The finish coat is applied in the field after the repair of any damaged coatings and the removal of surface contaminants.
• Additional surface preparation maybe needed to ensure finish coat adhesion or if re-coat window has been exceeded.
• This is the most prevalent method used.
• Only the primer coat is applied in the shop.
• The intermediate and finish coats are applied in the field after any damaged areas of the primer have been repaired and surface contaminants have been removed.
• Usually the damaged inorganic zinc primer is touched up with an organic zinc primer.
• Additional surface preparation may be needed to ensure intermediate and finish coat adhesion if contaminants are found or re-coat window has been exceeded on the intermediate coat.
