This is also from Compuserve MODELNET


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:

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 Composites.

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. Some are 1:1 mix, but commercial grades use other rates like 4:1. The proportion CANNOT be varied to accelerate the cure; using the wrong rate 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 does not bond extremely well with polyester fiberglass. Hobby grades include Pacer Z-Poxy and Smooth-n-Easy. Both work very well. An advantage is that the resin can be thinned with DENATURED alcohol to make it easier to spread. Denatured alcohol is also excellent for clean-up.

I recommend using epoxy resin, and the directions that follow illustrate this method.

B. Process:

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.

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 bits of CA and spurs of skin on your fingertips, and these will snag the cloth.

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, 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. After glassing the entire model, dust it clean and then brush on another coat of thinned epoxy. After applying, wipe off all the excess you can. This second coat helps to fill open wood grain and saturate any dry cloth. Let cure and finish sand. Be careful to sand lightly.

Now you're ready for paint!

C. Painting:

I prefer K&B Superpoxy over fiberglass, but I've used urethane, lacquer, enamel, and other epoxies successfully. The K&B primer works very well over glass, and the paint is really opaque and easy to apply. Nothing is more fuelproof than epoxy paint.

1. After sanding and wiping the airframe with a tack cloth, spray on a heavy coat of primer. 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 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. You want to remove most of 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.

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. That wasn't so hard, was it?

Jim Ryan 04-Nov-1995