BAS Data Sheet No 1
The Care and Protection of Polyester Gel Coats
Glass, carbon and kevlar reinforced composite structures
are widely used for the construction of modern gliders and aircraft.
During manufacture the composite structure is encased in a thin shell of
white pigmented polyester gel coat. This gelcoat serves two
main purposes; it enables a high degree of surface smoothness to be
achieved with an accurate profile thus optimising the aerodynamic
qualities, while at the same time protecting the structure from
ultraviolet radiation (UV) and water ingress. If left
unprotected, the gelcoat is slowly degraded by UV radiation causing the
surface to become yellowy and powdery as the gelcoat is oxidised.
Gelcoat is not waterproof and free water will permeate through the surface
and become absorbed by the structure. Extremes of temperature
will then cause the water to either vaporize or freeze, cracking the
Early gelcoats such as Schwabelack were relatively hard and had a long
life. However, its hardness meant that considerable time had to be
expended during the finishing stages in manufacture to achieve a
satisfactory finish. Faced with rapidly increasing labour
costs, manufacturers changed to a softer gelcoat (Vorgelat) which
significantly reduced the time spent in finishing the glider.
Unfortunately, the Vorgelat formulation used proved less satisfactory and
broke down in use. There is some doubt about the exact cause
but UV damage and water permeability are factors. Recent
advances in gelcoat technology have developed another, improved,
formulation: Scheufler Vorgelat T35 (Vorgelat is not a product but is
simply a German word that is best translated as 'first coat of paint').
T35 has been shown to be easy to work while offering good resistance to
water penetration, and should be more resistant to cracking. Most of the
glider manufacturers now use this gelcoat.
In order to help the gelcoat carry out the task of protecting the
structure, it needs some help from the glider owner. Firstly,
the only way to eliminate UV deterioration is to keep the glider out of
the sun. However, by the very nature of gliding this is not
practicable and when not actually flying it is of considerable help to
store the glider in a covered trailer or hangar. If this is
not possible, then the minimum protection should be a set of good wing,
tailplane and fuselage covers.
Moisture penetration of the gelcoat can best be minimised by a regular
machine buffing with a hard wax. The wax seals the pores of
the gelcoat preventing water absorbtion. It has a side effect
that it keeps the surface clean and shiny thus reducing contamination by
dust and dirt. The wax coating should be renewed annually in
temperate climates and more often in the harsh regimes experienced in
Texas, Australia, South Africa and southern Europe.
A further enemy of polyester gelcoats are chlorinated hydrocarbon
cleaning solvents. NEVER clean the gelcoat with MEK, Trichloroethylene,
Acetone or similar products. The use of any of these will
cause permanent damage to the gelcoat. While ethyl alcohol (iso
propyl alcohol) or petrol can be used sparingly to clean tape marks from
the gelcoat, it is much better to use a 'green' solvent such as De-Solvit
or good silicone free polishes such as Carlack or Lesonal. If
used regularly, these latter polishes will also provide some protection
against UV. Avoid the use of any polish containing silicones as it makes
it very difficult to re-finish a scarfed repair on the structure should it
ever be necessary.
There is also another hazard which can damage gelcoats and this is as a
result of high altitude flights. When flying at high altitudes
in wave, the glider structure and gelcoat become very cold. While
this does not affect the structural strength of the glider, the gelcoat
becomes hard and brittle in the sub-zero temperatures. If the
pilot now flies the glider at high speeds, or pulls full airbrake while
descending, the resultant flexing of the wings can cause chordwise cracks
in the gellcoat. If you do carry out high altitude flights,
then try and avoid sudden loads that flex the wings, particularly in the
descent while the structure is cold. Slow or stop the descent at
warmer levels to allow the structure to warm up before entering the
Remember the golden rules for gelcoat protection:
Don't leave your glider out in the sun for long periods without
Don't store your glider in a damp, moist trailer; either seal the
trailer interior during the winter leaving a bag of silica gel in the
cockpit, or ensure the trailer is well ventilated.
Don't put your glider away in the trailer wet; dry it off before
Ensure that the glider is regularly cleaned and machine buffed with
With a new glider, don't wait until it shows signs of gelcoat
deterioration before hard waxing.
Don't clean off tape marks etc with chlorinated hydrocarbons; use
alcohol or petrol or better still a cleaner such as De-Solvit or polish
such as Car Lack or Lesonal.
Don't use polishes containing silicones.
If flying at high altitudes in sub-zero temperatures, then avoid
flexing the wings by flying or operating the airbrakes at high speeds.
Slow the descent at the warmer levels to allow the structure to warm up
before entering the circuit.
Look after your gelcoat; tender, loving care will repay you in the long
run by maintaining the value of your investment and delaying the
considerable cost of renewing the gelcoat surface.
April 95 (edited Dec 2000)
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reproduced without permission