Polyurethane. Polyurethane is an excellent, hard
coating that has very good color and gloss retention. It is used
extensively over epoxy primers and is suitable in all but the
most aggressive environments.
Alkyd. This excellent coating system has been
used for many years on original equipment. Alkyd coating can
work in many areas of a WWTP, but not in immersion, high humidity
environments, or in exposures to strong alkalis, such as sodium
hydroxide or lime.
When selecting coatings, consider the entire system,
which includes surface preparation, prime coat, and intermediate
and finish coats. Good surface preparation is critical for a
successful coating application and good performance. However,
when dealing with existing surfaces, compromises may be required.
Surface Preparation. The preparation of the
surface is the most important variable in good coating
performance. Whenever an existing coating system has failed
significantly, it is recommended that the surface be cleaned to
bare metal. Recoating of an existing painted surface will
roughing of the surface to achieve a good bond of the new coating
Excellent guides for surface preparation are available
from MIL-HDBK-1110 and the Steel Structures Painting Council
(SSPC). Table 13 summarizes the SSPC Surface Preparation
Standards. These standards should be followed for all surface
preparation of metal substrates that are to be painted. In
addition to surface cleanliness, the surface profile (roughness)
must also be specified. Refer to the coating manufacturer's
requirements for the specific profile.
Although the SSPC standards are specifically addressed
to steel substrates, some of the standards are commonly used for
concrete substrates. SSPC SP12 specifically is not limited to