The Breakdown of Gelcoat
Gelcoat has some advantages over polyurethane paint (link to article) but does deteriorate over time.
Our technicians at XU Aviation are experts at refinishing gliders in gelcoat and if only an area needs to be refinished (because of a repair) we’re good at blending the edges of the new gelcoat into the original finish.
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 don't 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 Gelcoat?
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 isn't 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 doesn't 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 it's 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.
Polyurethane is the highly developed paint product that is on your car. It stands up well to the weather and I haven't 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 you're 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 the structure.
- For best results remove all the old gelcoat.
- Choose a refinishing system.
- Follow the manufacturer's instructions.
- Achieve original shape.