This large difference in initial cost of maintenance has made the choice of overcoating quite attractive to bridge owners. While overcoating may be a viable maintenance and economic option in some cases, the feasibility of overcoating a particular bridge should also be based on the relative potential performance of each maintenance option and the risk associated with potential early failure of overcoating systems. While an analysis of initial costs of various options will almost always point to overcoating, a life cycle analysis will often show full paint removal and application of a high durability coating system to be the most cost effective option, particularly for bridges or areas of bridges in marine environments or exposed to significant deicing salt application. As detailed above, the performance of a newly applied overcoating is expected to be highly dependent upon the condition of the existing coating over which it is applied. Overcoating over existing aged paints that are often brittle and loosely adherent can pose a significant risk of early failure and large scale disbondment.
Condition can include extent and distribution of rusting, and adhesion.
The extent of metal loss due to corrosion and the extent and distribution of paint breakdown are important in determining the scope of the work required to clean and overcoat the structure. Paint failure confined to specific definable areas is easier to clean and overcoat than general paint failure over the entire structure. If the coating lifts easily or crumbles at the tip of the knife blade, then application of overcoating paints should be considered high risk. Conduct a representative patch test of the new material over the existing material using representative maintenance painting practices. Allow this patch to weather several seasons and assess the compatibility of the systems.