The Breakdown Of Gelcoat
The polyester gelcoat on your
glider deteriorates with time. For various reasons, some
breakdown faster than others; however, they all deteriorate. Keeping
the surface sealed with wax and carefully removing any
oxidation on the surface helps prolong its life but eventually
they all break down.
Polyester continues to shrink
and harden as it ages. This process leads to stresses in
the gelcoat. The stress first shows itself as micro cracking
on the surface. This is because the deterioration is
accelerated on the surface as it is exposed to air, moisture
and ultraviolet light. These cracks quite often align with the
factory sanding lines running chord wise on the wings and
along the longitudinal axis of the fuselage. If at first signs
of micro cracking, the deteriorated surface is removed by sanding and
polishing carefully. The process will be slowed,
but the newly exposed surface will also break down.
There are a limited number of
times you can sand and polish, eventually you will wear
through. If you donít remove the micro cracks they will grow
into deep cracks. The cracking will continue to grow down to
the structure or actually grow into the epoxy matrix.
There is a Schleicher
Technical Note which states that from a structural standpoint,
cracking of the gelcoat down into the matrix (epoxy) is not
detrimental, but the unknown effects of exposing the fiber to
moisture and ultraviolet is to be avoided. The technical note
goes on to say that cracked gelcoat must be removed entirely
and a new surface finish applied.
Why Remove All The Old
Why remove all the old
gelcoat? Why not sand and fill the cracks with a sealer and
spray over the top? There are some products available to fill
and seal cracks in various finishes but they are made of
something quite different than polyester gelcoat so their
flexibility is different leading to future cracking.
If all the old cracked
gelcoat isnít removed, the flexing of the surfaces will cause
the cracks to grow up through the new finish and your new
finish will soon look just like the old cracked one! And
remember, polyester gelcoat always deteriorates with time, so the old stuff you just covered up is still there ready to
cause future cracks.
Removing The Old Gelcoat
Removing the gelcoat right
down to the fiberglass (or carbon) is tricky because the sand
paper doesnít discriminate between gelcoat and structure; the structure sands easier! An unskilled person can
severely damage the structure, which can be fixed if they are
knowledgeable enough to recognize that they did damage it.
Damage can sometimes be found from exposure which is caused by severely deteriorated
gelcoat. The technician must
recognize any damage and be able to repair it before the
refinishing process begins.
Applying The New Finish
Removing the old gelcoat is
60% of the work. Now itís time to apply the new finish.
There are at least 2 options for the new finish- polyester or
polyurethane. The original gelcoat was polyester, polyester
gelcoats are sprayed into the mold before the part is layed up
in the mold. Once the part has cured, it is removed from the
mold and out pops a nice part with a gelcoat surface that
needs very little work to complete. The seams (leading edges
of wings and top and bottom seam of fuselage) are smoothed and
sprayed with the same gelcoat and blended in to produce the
nice new gliders that we adore.
There are polyester
enamels available that appear to stand up better than the
product used by the original manufacturers. Polyester enamel
is sprayed onto the prepared surface, sanded to the original
contour then polished leaving a finish that has the same shape
and appearance as when it left the factory.
the highly developed paint product that is on your car. It
stands up well to the weather and I havenít seen a car with
crazed paint in a long time. A newer development in
polyurethane car paint systems is the clear coat which is
sprayed over the colour coats. The clear coat seems to be an
unnecessary step that only adds weight.
The process for polyurethane
differs from polyester in that much preparation work goes into
priming, filling and sanding smooth to the original contour
then when everything is right the finish coat is sprayed on.
If everything goes well youíre done, if there are any
imperfections you can polish them out. The appearance of the
polyurethane finish is different than polyester in that it has
"the wet look".
Generally, Polyester enamel is
better if you want the same look and ease of achieving and
restoring contours as original, and polyurethane is better if
you want a low maintenance shiny appearance.
Removing the old gelcoat is
lots of hard (dusty) work.
Be very careful not to damage
For best results remove all
the old gelcoat.
Choose a refinishing system.
Follow the manufacturerís
Achieve original shape.
> Go flying!! < <