failure. Controversy exists over the exact cause of this con-
dition, and many people use it as a catch all for unexplained
paint failures. They attributed the paint failure to dull planer
blades or excessive heat during planing. However, investi-
gations of reported mill glaze by FPL scientists showed that
other factors caused finish failure; scientists were unable to
duplicate mill glaze in the laboratory. FPL scientists found
three causes for paint failures that others had attributed to
“mill glaze”: (1) raised grain under a thin film, particularly
on smooth flat-grain lumber, (2) wood weathering prior to
application of film-forming finishes, and (3) moisture (usu-
ally water). These factors often occurred together.
Paint failure occurred because of raised grain on flat-grain
boards, particularly on species having abrupt EW–LW tran-
sitions. Planer blades tend to crush dense LW bands into less
dense EW that lie directly beneath them on flat-grain sur-
faces (Fig. 16–22a). Later, when these boards are exposed
to moisture, crushed EW absorbs moisture and rebounds,
which causes the surface LW bands to protrude from the
surface (Fig. 16–22b). A thin coat of film-forming finish
applied over a stressed flat-grain surface will crack as the
wood rebounds (Fig. 16–22c; see Paint Cracking). Failure is
most common on flat-grain siding finished with insufficient
film build of oil-based solid-color stain. Thin coatings of oil-
based solid-color stain and to some extent waterborne latex
stains are weak and do not withstand the stresses caused by
raised grain. These low-solids coatings provide only 0.03 to
0.05 mm (1 to 2 mil) of dry-film thickness, whereas a brush-
applied three-coat paint system (primer and two top-coats)
provides 0.10 to 0.13 mm (4 to 5 mil) of dry-film thickness.
Raised grain is less likely to occur with vertical-grain wood
because the EW–LW bands are perpendicular to the surface
and the EW is not crushed during planing.
Install flat-grain bevel siding saw-textured side out. Saw-
textured surfaces do not have LW bands compressed into the
EW. The saw-textured side is the side of choice for applica-
tion of penetrating semitransparent stains and film-forming
finishes. The film buildup on the saw-textured side will
be greater than on a planed surface, and the film will have
greater mechanical adhesion or “bite.”
If flat-grain siding must be installed smooth-side out, re-
move the planing stresses by wetting the surface, then allow
2 to 3 days for the surface to dry. Scuff-sand the surface
with 50- to 80-grit sandpaper and apply primer and two
top-coats.
Another paint failure that has been attributed to “mill glaze”
is peeling caused by wood weathering prior to applying
film-forming finishes (see Weathering, Effect on Paint Ad-
hesion). Water causes paint to peel (see Peeling and Flaking
and Water Blisters).Intercoat Peeling
As the name implies, intercoat peeling is loss of adhesion
between coats of finish, usually peeling of a new paint from
old paint (Fig. 16–23). It usually occurs within a year of re-
painting. Prevent intercoat peeling by ensuring that old paint
is free of dirt, mildew, and chalk prior to repainting.
Intercoat peeling can also result from allowing too much
time between applying primer and top-coat. If more than
2 weeks elapse between applying an oil-based primer and a
top-coat, clean the surface before applying the second coat.
If the primer (particularly oil-alkyd primers) has weathered
for several months, it may be necessary to re-prime prior to
applying the top-coats (see Testing for Adhesion).Chapter 16 Finishing of Wood
(a)(b)(c)Figure 16–22. (a) Cross-section view of flat-grain
southern yellow pine showing dense LW bands
crushed into less dense EW directly beneath them; (b)
raised grain caused by rebound of LW bands following
wetting; (c) a thin coat of film-forming finish applied
over a stressed flat-grain surface will crack as the
wood rebounds.Figure 16–23. Intercoat peeling of paint, usually
caused by poor preparation of old paint surface or
excessive weathering of primer prior to application of
top coat.