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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 gelcoat.

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 circuit.

Remember the golden rules for gelcoat protection:

  • Don't leave your glider out in the sun for long periods without protection.

  • 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 de-rigging.

  • Ensure that the glider is regularly cleaned and machine buffed with hard wax.

  • 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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