The Care and Protection of Polyester Gel Coats
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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 optimizing 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 oxidized. 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 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 labor costs, manufacturers changed to a softer gelcoat (Vorgelat) which significantly reduced the time spent in finishing the glider. Unfortunately, Vorgelat 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 gelcoat, Scheufler T35, which is both easy to work and offers 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, tail plane and fuselage covers.
Moisture penetration of the gelcoat can best be minimized by a regular machine buffing with a hard wax. The wax seals the pores of the gelcoat preventing water absorption. 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 a good silicone free polish such as Car lack 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 air brake while descending, the resultant flexing of the wings can cause chord wise cracks in the gelcoat. 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 air brakes 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.
So that you can keep your gelcoat in pristine condition, RD supply a kit, P/No T35/KIT, containing a small quantity of Scheufler T35 Gelcoat with SF2 Hardener that is ideal for touching up small chips on the surface of your gelcoat. Before mixing the T35, clean and degrease the area of the chip with iso- propyl alcohol. Then using a small piece of 240 grade wet and dry paper, carefully abrade the edges of the damaged surface of the chip. Be careful not to damage the surrounding gelcoat surface. Once the hole is clean, mix a small quantity of T35, carefully judging a ratio of 98 parts of gelcoat to 2 parts of 2% SF2 hardener. Mix thoroughly with a small stick such as match stick, taking care that you do not entrain too many air bubbles into the mix. Carefully dribble a small quantity of the well mixed viscous gelcoat into the cleaned chip cavity, leaving the surface slightly proud. Let the mixture harden for at least 12 hours at a minimum temperature of 18øC.
Once hard, carefully rub down the proud surface of the repaired area with 320/400 grade wet and dry wrapped around a wooden or hard rubber block. When it is nearly level with the existing surface, change to 600 grade and continue to rub down very carefully watching very closely to ensure that you do not rub through the existing good surface. Once it is flat, change to 1000/1200 grade paper and carefully finish the whole area. Dry and then polish with Car lack to restore the surface finish.
Take care with the T35 hardener; it is a hazardous liquid (organic peroxide) and is particularly dangerous if it gets into the eyes. If you accidentally get some in your eyes, wash with copious amounts of water and seek medical advice without delay.