There are many ways to apply fiberglass cloth to a structure. The most common methods use either polyester or epoxy resin and catalysts. Some of the resins require unequal mix ratios, like 1 part epoxy to 2 parts hardener, which is difficult to measure accurately. I also dislike polyester resin because it really smells foul. For all these reasons, I now use Pacer Technology’s Z-Poxy Finishing Resin (#PT40). It is an equal-mix product that has little to no smell and is easy to work with. Cure time to a tacky consistency is about 30 to 45 minutes. After about two hours you can handle it, and after four to five hours, you can sand it or apply another coat. Even if you have never done this before, it is really easy to do.
Let’s get started
Some items you will need are paper towels, 1-inch hardware-store chip brushes, something stiff to use as a “squeegee” (a credit card works great, as do plastic-coated playing cards), a hobby knife with no. 11 blades, Z-Poxy Finishing Resin, and some denatured alcohol. A flat surface, like an old table or workbench, covered with newsprint or some other disposable covering is excellent for setting parts on. You will also need some 100- and 220-grit sandpaper to smooth out your prep work. A rubber sanding block, available at most hardware stores, will help enormously. Oh yes, and some 5-oz. disposable drinking cups will be useful for mixing the resin. Keep in mind that different-weight fiberglass cloths (from 3/4 to 3 oz.) work differently. The heavier the cloth, the more stubborn it may be to lay down properly—be patient. Whether it is a one-piece or a two-piece wing, pretend that it’s divided into four quadrants: top left, top right, bottom left, and bottom right. It doesn’t make a difference which section you start with. If you have detachable ailerons and flaps, remove them and do them separately. Now, let’s get down to business.
Before you start the first panel, make sure that the whole wing has been finish-sanded with 220-grit sandpaper and then vacuumed clean. Do the other three panels in the same way. If your balsa wing has any dents in it, spray the dented area with a little water and gently rub it into the dent. Let the area dry out; the wood grain will lift slightly, causing the dent to vanish like magic!
The general idea here is to have smooth, flat, ding-free surfaces over which to lay your fiberglass cloth. As far as fiberglass cloth goes, my basic guide to what weight to use is directly related to airplane size. For models with wingspans in the 80-inch range, you can use anything from 3/4- to 1 1/2-oz. cloth. Anything 100 inches and over, I use 2- to 3-oz. cloth. You get the idea. To find where to buy your glass cloth, check hobby shops or large distributors. You can also shop around your local boat/marine hardware stores for the cloth you need. Now, measure your wing panels and allow at least an inch of waste material all the way around. After cutting your first piece of cloth, lay it on top of the wing panel and rub your hand over it to make sure that there is nothing under it, as any speck of sanding dust will ruin the finish.
The glass act
For the first coat of resin, I add about 5 to 10 percent alcohol to the resin. This is not really recommended by the manufacturer because of the variances of alcohol used. But it helps the cloth lie down easier without pulling it out of shape while applying the resin. When you apply the second coat of resin, however, use it full strength. Mix the resin parts together completely, add the alcohol, and mix again until the alcohol is fully integrated.
Lay the cloth down over the bare balsa, being sure that it lies down completely smooth. Pour a small line of the mixed resin down the center of the wing panel span-wise, and spread it out from the center to the leading or trailing edge—it doesn’t matter which—then do the rest. Keep a disposable drinking cup handy to squeegee off excess resin from your credit or playing card. Paper towels are nice to have around, too, as well as a 1-inch chip brush to brush down the edges and areas that you missed. Now, spend some time going over those areas; fixing it now makes life a lot easier later on.
By this time, your hands will feel as if you just handled a large ball of cotton candy—nice and sticky. Wipe yourself and any tools you wish to keep clean using a paper towel and some alcohol. Trust me—once you have completed the first panel, you will find the other three will go much faster. I usually do the top-left panel followed by the top-right panel and then let cure. I trim the edges with a no. 11 hobby blade and then lightly sand the edges all the way around. I flip the wing panels over and apply cloth to the other side. Don’t build up too much resin near any edges because, when it cures, you will just have more resin to sand off.
After you’ve done all four panels and they’re all fully cured, revisit the first panel and very lightly sand and fix any little imperfections or dings that you might have picked up. If you have any wrinkles, bubbles, or brush hairs, now is the time to sand them away. If you must sand down so far through the cloth and resin that it leaves a bare spot, don’t worry. You can cut a small piece of cloth and put it down with the second coat of resin, or if it’s small enough (like the size of a quarter), just ignore it and put down the second coat of resin directly over the bare wood. Go over the other three panels and do the same thing. As for the wingtips, I simply trim close to the wing, apply resin, let cure, and then sand. If needed, I’ll add scrap cloth to cover any bare spots.
Once you have all four panels done with two coats of resin and they are fully cured, you can use the sanding block to sand the wing smooth. You will notice that the second coat of resin has cured fairly shiny. This is because it just lies on top and does not absorb at all into the wood or the glass cloth. Use 100-grit sandpaper over the shiny surfaces to “break” the shine, then switch over to a finer-grit sandpaper, like 220, to finish-sand the whole panel. Remember that when you sand the cloth, be careful not to go through to bare wood. The difference now between a good wing surface and an outstanding one is how much time you spend sanding the wing.
When you have the entire wing finished as mentioned above, you are just about done. For me, the next part is the final step. Find a well-ventilated area, and get some Rust-Oleum Automotive gray primer (an average rattle can will cover about 4 square feet) and lightly spray the wing panels. The primer will expose any flaws left in your fiberglass surfaces. Fill any pinholes or dings using a lacquer spot putty or Bondo, let dry, and sand and primer again. From here, there are many ways to finish the wing, but that’s another story.
For strength, I always glass the wheel-well interiors.
Our current project is a giant-scale Grumman Hellcat. As you can see, the wing has been finished nicely with fiberglass cloth and resin and is ready to prime. The fuselage was finished earlier and already wears a coat or two of primer.
The same basic technique is done for all control surfaces, as shown here. The parts are set aside until the resin cures, then the excess cloth is trimmed away and sanded smooth using sandpaper.
Photos and Text by Denny DeWeese