MIL-HDBK-1110
insoluble," if not. Solvent soluble coatings are generally not
compatible with coatings having strong solvents such as epoxies
and polyurethanes.
A more system-specific way to determine compatibility
of a new coating with an existing one is to apply a small patch
of the new paint over the old one. If any of the
incompatibilities described below exist, it will become visually
apparent on the patch within a few days. Incompatibilities
associated with differing mechanical properties (e.g., a more
rigid coating over a more flexible one) or sensitivity to
alkaline conditions occur in a longer timeframe and are also
discussed below.
2.4.1
Bleeding. Bleeding (staining) may occur when a coating
with a solvent is applied over an existing bituminous (coal tar
or asphalt) paint or pavement. The solvent dissolves the
bituminous material and permits it to spread through the topcoat
to cause a brown surface discoloration. This normally does not
adversely affect the film properties but produces an unsightly
appearance.
2.4.2
Disbonding of Old Paint. Strong solvents in a topcoat
may penetrate the existing undercoat and reduce its adhesion to
the substrate. This may then result in disbonding of the total
coating system from the substrate.
2.4.3
Topcoat Checking. An incompatibility may occur when a
relatively rigid topcoat is applied over an existing flexible
coating. If the topcoat checks (cracks in the topcoat only) to
relieve the stress.
2.4.4
Poor Adhesion of Latex Topcoats to Enamels. Problems
are frequently encountered in obtaining good bonding of latex
topcoats to chalky surfaces or smooth fully-cured alkyd enamels.
There is usually insufficient organic solvent in the latex
topcoat to dissolve sufficient enamel to bond tightly to it. It
may be necessary to first lightly sand the enamel to provide more
texture for adhesion and/or apply an oil-based primer as a tack
coat, before applying the latex topcoat.
2.4.5
Oil-Based Paints Applied to Alkaline Surfaces. Moist
alkaline conditions cause a slow breakdown of oil-based paint
films. The chemical reaction is called hydrolysis or
saponification. The rate at which this occurs and the resulting
rate of coating deterioration depend upon the environmental
conditions and the specific formulation of the materials.
However, in time oil-based coatings applied to alkaline surfaces
will delaminate and peel.
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