PVA as a mold release
By Tom Pawlak
PVA (polyvinyl alcohol) is a water-based liquid plastic that can be
applied to waxed molds to prevent molded parts from sticking. PVA is not always required.
Often laminators rely solely on mold release wax to create a surface contaminant on the
mold that allows parts to be removed. However, on complicated molds and on new molds, when
the risk of sticking a part in a mold is greatest, PVA can be applied over a waxed mold to
minimize the chance of sticking a part.
PVA can be applied by brush, paint roller or with spray equipment. PVA
often leaves a texture on the surface of the mold after it dries. This texture is
duplicated or molded into the part produced in the mold. For parts that will be painted
later, this texture is not an issue. However, for parts that rely on the mold surface to
create a smooth final finish, the texture can be a problem.
To achieve the smoothest and least textured PVA coating, apply it with
spray equipment. The preferred application technique is to spray a couple of very fine
mist coats a few minutes apart before applying a more generous wet coat over the entire
mold. This technique minimizes the likelihood of getting "fisheyes" or
imperfections in the coating and produces a very smooth PVA release coating.
For amateur builders, PVA can be difficult to purchase in small
quantities and often requires delivery from out of town sources that may include a
hazardous shipping fee. It is most often sold in liquid form, which requires spray
equipment for best results. However, there are sources of PVA in aerosol form which makes
spray application convenient for small part laminators.
Hair spray as a mold release
A number of years ago, my friend Jack Anderson used hair spray in place
of PVA as a mold release. He had built a mold to make air vents for his sailboat. The mold
had minimal draft angle (relief angle which allows parts to be removed from a mold after
the laminate cures). Ordinarily in our Gougeon Brothers shop, we would prepare molds with
multiple coats of paste wax without applying PVA. With this mold, however, the parts were
difficult to remove and it was obvious that a back-up mold release would be required to
make removing parts easier.
Jack tried brushing PVA into the mold but it ran down the sides and
puddled in the bottom. It also fisheyed on the sides of the mold, creating a disappointing
effect.
Reasoning that hair spray is water-based plastic (because it can be
washed out with water after it has dried), Jack decided to try it as a mold release on his
vent mold. He misted on the first couple of coats and then applied a heavier coat. Using
the hair spray as a back up mold release, Jack was able to release the parts from his mold
with less difficulty.
It's been years since Jack showed me his hair spray/mold release
trick but recently I found a need for PVA on a mold I was building at home. Since the plug
for the mold was a bit complicated, I felt I needed some insurance to make sure the mold
would release from the plug. I decided to give Jack’s hair spray trick a try.
First, I waxed the mold plug with five coats of Meguiar’s Mirror
Glaze 8 Maximum Mold Release Wax™ in preparation for applying the hair spray/mold
release. I could have also used Meguiar’s Mirror Glaze Automotive Paste Wax™ as
we have used it successfully as a mold release wax on molds that were not exposed to
temperatures over 110°F.
Then I selected an extra heavy hold hair spray, choosing it rather than
a lighter hold hair spray because it would be less watered down. I also chose an aerosol
rather than a pump type because the aerosol would work well for misting the mold. The good
news is my mold released effortlessly from the plug. Since then, we have tried two other
heavy-duty hair sprays and both types substituted nicely for the traditional PVA mold
release.
Removing PVA before painting
PVA transfers to the laminated part when the part is removed from the
mold. It is important to remove the PVA from the molded part before painting because it
will cause paint adhesion problems and surface imperfections in the paint if left on the
surface. We remove PVA with warm water and an abrasive pad before sanding in preparation
for paint.
Test first
If you decide to use hair spray in place of PVA for releasing cured
epoxy, be sure to test it ahead of time on a non-critical surface. Keep in mind that PVA
is normally applied over waxed surfaces and is not particularly effective as a mold
release on its own.
Speaking of wax
For amateur builders, automotive paste wax made with carnauba wax can
often be substituted for mold release wax. Meguiar’s Mirror Glaze Automotive Paste
Wax is one brand that has worked well for us. Avoid liquid car waxes, as they are not as
effective and can cause release problems. Apply at least three coats before you use the
mold for the first time, then apply another coat after each use Test any wax you plan to
use to make sure it will release effectively before using it on critical projects.