COVERING MODELS WITH FIBERGLASS CLOTH
One of the highest quality finishes for fully sheeted balsa surfaces is painted fiberglass cloth. Please note that open structures like unsheeted wings CANNOT be covered with fiberglass. Fiberglass provides a ding-resistant surface, increases the flexural and torsional strength of the airframe, and adds minimal weight.
A. Materials: The required materials can be purchased at most well-equipped hobby shops or by mail from companies that specialize in composite construction.
1. LIGHT glass cloth. For most work, you want .56 or .72 ounce/sq yd cloth. Light cloth is available in bulk quantities from several companies, including Composite Structures Technology, Fiberglast Development Corp and Aerospace Composite Products. Sig sells small packages through hobby shops.B. Process: The application process is easy, but if you add too much resin, you can make a big mess (and a heavy airplane). If things don't seem to be going well, peel off the cloth and use a heat gun and paper towels to clean off as much of the resin as possible. You can always try again if you let the resin cure, sand the surface and start over.
2. Resin. There are two main types of finishing resin: polyester resin and laminating epoxy.a. Polyester Resin: This resin uses a catalyst for starting the curing process. The advantage of this is that the cure rate can be accelerated by adding extra catalyst (within limits). The material has a strong odor, and will not cure over epoxy. It also attacks bare foam. The catalyst is extremely dangerous; contact with eyes will cause blindness. K&B polyester resin is the most popular hobby grade.
b. Laminating Epoxy: This is NOT the same thing as the adhesive epoxy used for assembling models. Laminating and finishing epoxy form a very hard and sandable surface, not the rubbery surface of adhesive epoxy. Epoxies use a proportional mix of resin and hardener. Hobby grades are usually a 1:1 mix to simplify measurement, but commercial grades are usually 4:1. The proportion CANNOT be varied to accelerate the cure; using the wrong ratio results in a soft, rubbery lay-up that will never fully cure. Epoxy has much less odor than polyester resin and will cure over nearly any surface, although it doesn't bond extremely well with polyester fiberglass. Hobby grades include Pacer Z-Poxy and Hobbypoxy Smooth-n-Easy. Both work very well. An advantage of epoxy is that the resin can be thinned with DENATURED alcohol to make it easier to spread. I recommend using epoxy resin, and the directions that follow illustrate my method for epoxy glass covering.
1. You must start with a high quality surface. The cloth will not conceal errors. Fill or steam out all dings and finish sand the airframe with 240 grit abrasive and dust it off before proceeding. The finished product will be no better than what you start with here.C. Priming: Epoxy fiberglass will accept a wide variety of paints. I've used K&B Superpoxy, urethane, lacquer, and enamel. K&B primer works very well over glass, but is somewhat expensive. I've gotten very good results with inexpensive sandable primers like Plastikote from auto stores. Just make sure the primer will be compatible with your color paint.
2. Wear latex or vinyl gloves. This is as much to protect the cloth as it is to protect you. After sanding a model, you'll have rough spots on your fingertips, and these will snag the cloth. This cloth is as sheer as gauze, and it's easy to snag.
3. Lay the cloth out over the area to be covered (I recommend starting with the underside of a wing, as it's about the easiest to cover) and cut the cloth to size, leaving about 2" of extra cloth around the perimeter. Brush the cloth down with a DRY hair brush. This smoothes out any wrinkles and imparts a static charge that will make the cloth cling in place.
4. Mix the resin: For Z-Poxy and Smooth 'n' Easy, I use 1 part resin, 1 part hardener, and 1 part denatured alcohol (don't use rubbing alcohol, which is diluted with water). Mix thoroughly. The alcohol won't effect the strength of the final product. It's very volatile, and it'll evaporate well before the resin starts to cure.
5. Brush the thinned resin on so that you fully saturate the cloth. I recommend an inexpensive hair brush, rather than the stiff nylon brushes, which tend to snag the cloth. Brush application is the advantage of thinning; you can spread the resin quickly and easily. Leave the excess cloth around the edges hanging loose; you'll remove it later. The cloth will follow compound curves like wingtips very well; just tug at it lightly as you saturate it, and it'll smooth out perfectly.
6. After you've saturated the cloth, go back over it with cheap toilet paper and blot the surface. You need to remove all excess resin, as it adds unnecessary weight. Look for shiny areas and blot them until you have a uniform dull surface. If you see any whitish areas, you didn't apply ENOUGH resin in that spot. Re-apply to that area and blot again. Handy Tip: I like to lightly heat the surface with a heat gun to make the excess resin easier to blot.
7. Let the epoxy cure; overnight is best. Trim off the excess. If you wish, you can just sand around the perimeter with 240 grit and the excess will come loose without trimming (neat, huh?). Sand the surface lightly and then do the other side of the wing. You can overlap the cloth at the leading edge, but this isn't critical. Don't worry about trying to get the cloth wrapped around the trailing edges; just let it trail off. After finish sanding, the seam will be invisible.
8. While finish sanding with 240 or 320 grit, be careful to sand lightly. The glass is thin, and you can sand through it if you're not careful.
9. After covering the entire airframe, I like to go back with a second application of thinned resin, especially around the edges. Use the heat gun again and wipe off as much of this resin as you can; all you're trying to do is seal the wood for the priming step. Now you're ready for paint!
1. After sanding and wiping the airframe with a tack cloth, spray on one or two heavy coats of primer. Neatness isn't critical; bear in mind that you're going to sand nearly all of it back off.D. Painting: I've added some notes on paints suitable for electric models. When I built glow models, I was very happy with the results I got with K&B Superpoxy; it's reasonably easy to work with, tough, and utterly fuel-proof. But for electric models, there are some lighter and easier to use options.
2. After the primer dries, go over the airframe looking for open wood grain that's showing through. If you don't see any, you probably applied too much resin <grin>. The easiest way to fill open grain is to mix up a cup of diluted lite spackle like Model Magic or Red Devil. Add a little water and mix the spackle until it has the consistency of whipped cream. Then use a credit card to sqeegee the mixture over the airframe. I usually work across the grain, and the open grain can be filled really quickly. Let this dry for a couple of hours before going on to wet sanding. Isolated open grain and dings can also be filled with a styrene putty called "Squadron White Putty". It comes in an orange and white tube and can be found at most hobby shops, especially those that cater to model railroading. If you can't find white putty, auto body spot filler is an acceptable substitute. Apply the putty with a spatula and let it dry.
3. After the putty or filler dries, wet sand the model with 320 or 400 grit wet/dry. You want to remove nearly all the primer, which is there just to fill the cloth weave, but be careful not to sand through the cloth, which is very thin. Extra primer only adds weight, so try to remove as much as you can. When you're done, the model should just have a "dirty" look from the remaining primer.
4. After finishing the wet sanding, wipe the airframe with a damp cloth and wipe dry. You now have a perfect surface, ready for the color coats of paint. If you're really going for a contest-quality finish, you can apply a second light coat of primer and wet sand the airframe once again.
1. Aluminum Paint: Normally, natural aluminum is one of the hardest colors to simulate on a model. But Krylon Dull Aluminum makes a terrific weathered aluminum finish. Also, it's cheap, readily available, and extremely easy to apply. It has excellent opacity, and two light coats is normally sufficient to get a perfect finish. To get a nice, even base coat, I even mist on one or two light coats of dull aluminum on models that I'm going to paint another color. This allows me to see any defects, and it allows me to use fewer coats of the other colors.
2. Military Paints: I've never found a better paint than the Floquil military paints. As the people at Floquil are fond of claiming, the pigments are ground finer than those used to paint a Ferrari. They air-brush beautifully, thin and clean up with regular mineral spirits, and the color matches are perfect. To use Floquil, I add enough thinner to fill the bottle almost up to the top (think they planned it that way?), which is about 25%. This consistency works very well with my air brush. You want to mist on an initial coat, and then apply one or two wet coats. The key is to spray the paint just heavy enough so that it flows out, rather than having a dry grainy appearance. You can force-dry Floquil with a heat gun. After completing the painting and detailing, Testors Dull Cote will help protect the finish.
Good luck! Remember, there's nothing like a painted finish.