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.
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 Structure Technology, Fiberglast Development Corp and Aerospace Composite Products.
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. 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.
2. Wear latex or vinyl gloves. This is as much to 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. This 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 paper towels 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.
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, overlapping the cloth by around 1/2". After finish sanding, the seam will be invisible.
8. While finish sanding, be careful to sand lightly. The glass is so thin that you can sand through it if you're not careful.
Now you're ready for paint!
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 Krylon from auto stores. Just make sure the primer will be compatible with your color paint.
1. After sanding and wiping the airframe with a tack cloth, spray on a heavy coat of primer. Neatness isn't critical; bear in mind that you're going to sand nearly all of it back off.
2. After the primer cures, 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 ;-). The open grain can be filled with a styrene putty called "white putty". It's made by Testors, comes in a tube that looks just like their model airplane glue, and can be found at most hobby shops. Apply the putty with a spatula and let it dry.
3. After the putty dries, wet sand the model with 400 grit wet/dry. You want to remove nearly all the primer, which is there just to fill the cloth weave, but be very 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.
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.
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.
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.
Good luck! Remember, there's nothing like a painted finish.