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Gel
coat applications should be done under careful, well-planned
procedures. Listed below are a few standard rules to follow
when you apply a pigmented coating.
1.
As in waxing, start gel coating the mold in the same general
area for each pass and stop in the same place. This will
keep you from losing your place as you progress around the
unit.
2.
Apply the gel coat in wet passes, about 5-7 mils per pass.
This will allow the coating to release air and will assure
a porosity-free surface. If dry passes are applied, the
gel coat will air-dry instantly, will not release air and
could cause "alligators".
3.
The total millage should end at no more than 25 mils wet.
Coatings will shrink at different rates but should leave
a final layer of about 21-23 mils dry. Heavier builds could
result in cracking and crazing of the gel coat as the mold
becomes hot in production. Four wet passes of 5-7 mils
should yield the final required millage.
4.
The spray gun should be held 12-18 inches away from the
surface of the unit.
5.
By spraying perpendicular to the surface and moving the
gun in a cross pattern you will maintain an even layer of
coating over the entire surface. Pre-release occurs when
the coating shrinks at unequal levels. This can be aggravated
by heavy and light layers of coating.
6.
Clear tooling gel coat is a process used more and more in
today's tool shops. Being able to visually inspect the first
15-20 mils of gel coat prior to applying the back-up gel
coat and laminate is reason enough to consider this process.
After applying 15-20 mils of clear tooling gel coat, inspect
the work. Then apply the desired color of the mold on the
back side of the clear at about 7-10 mils. If you find air
entrapment, dirt or any other defect in the clear, remove
it from the mold and take a corrective measure, such as
lowering the catalyst levels, etc.
Some
benefits of clear tooling:
1.
Higher gloss retention due to the higher resin solids in
the clear.
2.
Easier releases due to a tighter surface created by the
lack of fillers such as pigments.
3.
Non-noticeable repairs. Most pigmented systems become noticeable,
even when the same batch and product are used, due to the
difficulty in matching the cure times, cover and hide of
the unit being repaired. Clear is clear. There are no cover
and hide problems associated with clear.
4.
Clear can be applied with a hydraulic, "airless"
piece of equipment. A pigmented tooling gel coat should only
be applied with an "air aspirated" piece of equipment
and never with an airless one, due to the high degree of
air impregnated into the coating by hydraulic atomization.
5.
Less severe wax build-up. There will be fewer such problems
than on a pigmented surface because of the fewer wax cycles
required for the mold.
Two
minor problems are associated with clear tooling gel coat:
1.
When applying the coating, it is very difficult to visually
determine how much coating is being applied to the unit.
By
making several passes with the gun on a test panel and milling
it, you can establish the amount of material being applied
per pass. Keep your passes as rhythmic as possible and count
your passes. Set the equipment at a level that will give
you 5-7 mils per pass at your speed.
2.
If sanding is necessary, use a rubber squeegee to wipe the
surface being sanded. This will dry the surface instantly,
thus allowing you to see how deep you are sanding.
The
larger and, consequently, the more expensive the mold is to
build, the more valuable clear tooling gel coat becomes to
the tool-builder. New procedures and techniques are surfacing
very rapidly in the fiberglass industry. Clear is definitely
here to stay.
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