 | FAQ's |  |
Frequently asked questions about fabric covering and painting model aircraft.
Watch for the"  "symbol. This page will be updated regularly as additional information is requested. We have all seen one model that stands out in our memory as the perfect model. My personal favorites are the fabric covered ones such as the Waco, Stearman, Taylorcraft, and the ever loving Cubs. After spending most of my life at ¾ the speed of sound, I like to slow down when I have the chance. Whenever we see those museum pieces on the flying field the first question that comes to mind is usually, "how did you get such a beautiful covering job". The answer is a combination of materials, techniques, and experience. We will try to provide a few answers about the polyester method and hopefully answer a few questions in the following article. What we don't want to do in modeling is re-invent the wheel, or re-invent what the full scale builders have known for more than half a century. Why use 30 year old technology? Because it works and works well! It has been improved upon by experts until it does the job we want without fail. What we will discuss here is a process invented by EAA (Experimental Aircraft Association) hall of famer Ray Stits. It is sometimes referred to as the Stits process or more recently as the "Poly-Fiber" build up process. The Poly-Fiber build up process is a polyester process and chemically bonds until you have a unitized layer when you finish. It doesn't matter if we are talking about the first layer of Poly-Brush or the last layer of Poly-Tone. They all bond together. Can I use other paints on Poly-Fiber or polyester fabric? Yes, absolutely! All we are suggesting is, if you stick with (pun intended) all Poly-Fiber products, they will bond and give you a better product. All Poly-Fiber paints are polyester vinyl and remain flexible but don't shrink. Dope shrinks throughout its life as it is designed to do. Epoxies are not as flexible and may crack on open surfaces such as open bays between wing ribs. On any Polyester fabric, nitrate dope may shrink and peel away, unless the dope has completely encapsulated the thread. Dope was great on silk or linen because it was able to attach to all the tiny fiber around the thread. Polyester is woven from monofilament line, very smooth monofilament line. If you start with dope, stick with dope. If you start with Poly-Fiber stick with Poly-Fiber. You can paint, prime, prep or ready the fabric in any way you have done it in the past. Just remember, any problems or flaws you may have had then, may present themselves again, such as chipping, peeling, or cracking of the paint. Fabric on a wing does three things for us. First and foremost, it provides the airfoil to make this magic of flight happen. There are a myriad of ways to get away from covering a wing with fabric: foam covered wings, plastic coverings, and etc. But, then it just doesn't look the same and almost never replicates the aircraft we're trying to model. Last but not least, none of the plastics coverings provide the strength needed on today's larger models. Why invest hundreds of dollars on a model kit, several hundred dollars on an engine, (I know there are no wives reading this, right!) and then skimp on the structure of the airframe. The second benefit of fabric on a wing is it provides a monocoque construction. Just look how much stronger and less flimsy a wing is after it is covered! Now try Poly-Fiber and that strength will more than double and it will stay put. Poly-Fiber been used on aircraft for more than 30 years and we will try to explain a few tricks. Polyester has a memory when it is woven and that is why it shrinks when heated. It is returning to its original position. The question is always HOW MUCH WILL POLYESTER SHRINK? That is where we get confused. It depends on the fabric, the weave, the manufacturer, and last but certainly not least, the temperature at which we shrink. Poly-Fiber shrinks 10 to 12% at 350°F. Most polyester will begin to shrink at 200°F., this will begin to take out the large wrinkles. You should either calibrate your iron with an accurate thermometer (ours is marked with "shrink rates" alongside the temperature) or use the new Coverite iron. It does an excellent job of temperature control. Just dial the temperature you desire and the iron delivers within ±3°F. Sort of like "dial a shrink". Clear epoxy will prevent any grain showing and will prevent the Poly-Tak and Poly-Brush from soaking into the wood. This way the Poly-Tak won't soak into the wood and the grain positively won't show through the fabric. It is recommended to use Poly-Tak cement. Because it dries very quickly, we pour it into a small container and work from there. You should only apply the Poly-Tak over 12 to 24 inches at a time or it may dry before you apply the fabric. As you become more familiar with the procedure you will modify the methods to suit you. You should apply the fabric over the wet cement and wick it up through the fabric. Don't worry about getting all the wrinkles out or "stretching" the material over the airframe. The fabric will shrink more than enough, and it won't bubble or sag as the temperature changes. It will pull over compound curves and not pull away later. If you happen to get wrinkles around the wing tip and can't pull them out, try the following technique. Heat your iron to 300 degrees and shrink the fabric along the tip as you glue it down. After the fabric is attached to the airframe, heat the iron to 200°F and smooth the edges down. Then, increase the temperature in 25° increments to shrink the fabric. You can stop at any time the fabric feels tight enough or go to 350° for the full pull. The Poly-Fiber will shrink 10 to 12% at 350°. Yes, as we said earlier, any method you have used in the past will work. But remember, Poly-Tak cement is a polyester base and will bond to the fabric. It will also bond fabric to fabric as in a patch or overlapping seams. Poly-Tak is a fast drying adhesive you will find many uses for besides attaching Poly-Fiber to the airframe. Poly-Fiber fabric can also be lifted and re-applied if you used Poly-Tak to attach it. Just use a cloth wet with MEK over the area you want to lift and the Poly-Tak will soften and the fabric can be lifted and re-applied. This is the third thing fabric does for us. It provides a surface for our paints. The manner in which the surface is prepared determines the final finish. In staying the course with Poly-Fiber we recommend filling the weave with Poly-Brush. The first coat should be brushed on in order to penetrate the fabric and provide a secondary adhesive to attach the fabric. Anywhere the fabric touches the airframe, the Poly-Brush will then attach the fabric. Be careful not to drag the brush, as it will leave brush marks. Make two passes with a full brush and move on.The secondccoat of Poly-Brush will be sprayed. Yes, but you won't have a chemical bond. You will have to hope for a mechanical bond. Which, by the way, is why some of you are having problems with the paints peeling. You are using dissimilar paints without enough surface abrasion to adhere. Yes, enamels, polyurethanes, and vinyls can be used over Poly-Tone. Just remember, it's the solvents in lacquers that make them bond. It's the solvents in Poly-Tone that make it bond. Will they react with each other? Always test the trim if it is a different brand than the base color! It's always better to stay with one brand or composition of paint. Yes, polyurethane can be applied over almost any paint or covering. Just remember to test it if you're not sure. Yes, the methanol in our fuel isn't a strong enough solvent to affect a polyester vinyl. Neither is nitromethane in reasonable amounts either. What may be a problem is if you spill gasoline with ethanol mixed with it. Gasoline isn't a problem, ethanol is. We have tested Poly-Tone with 20%nitro with no ill effects. Good question! You can either obtain a chemical bond with FC-900 or provide a mechanical bond i.e. 320 grit sandpaper or. FC-900 or Feather Coat, is a bridge between composites and Poly-Tone. Besides making a great filler it provides the chemical bond between a non polyester surface and the polyester vinyl paints. There is another way which is a little more trouble but an excellent bond. After applying a two-part epoxy primer, allow it to cure to touch. Then, apply the Poly-Tone color. The two will bond and will provide a surface for future coats to bond to. Be sure to only allow the primer to cure to touch, not completely dry. The way they are applied on full scale aircraft is with Poly-Brush. After the fabric is heat shrunk and the first coat of Poly-Brush is applied , the stitching is completed. On a model, you can stitch the fabric with Oral-B Dental Floss. It is about therright size and is waxed like the thread the big boys use. The next step is to brush a very heavy coat of Poly-Brush along the top of the surface where you want to attach the finishing tape. Apply the tape along the wet Poly-Brush and tap the surface forcing the Poly-Brush up through the tape. You want to see a void free laminate (No bubbles). Danny Gayhart of Poway California showed us a new method of fake stitches on his ¼ scale Cub. Danny attached the tapes with Poly-Brush, then applied the stitches with White Glue on top of the tapes. While completely opposite the full scale method, they look beautiful. Any white glue or aliphatic resin will work. Remember, you just want the hint of a stitch, not an obnoxious bulge in the tapes. For applications such as a vertical fin with a sharp radius, you may want to apply the Poly-Tak just along the very edge. Apply the tape and allow it to dry. Yes, the tape is perpendicular to the surface. Now, take an iron at about 300 degrees and iron the tape down along the surface. After the tape is flat, brush a coat of Poly-Brush underneath the best you can. Trying again for that void free laminate. If the Poly-Brush doesn't have the adhesiveness to hold the sharp radius, use Poly-Tak. Just be careful not to let it ooze out around the edges. The Scale Stits Covering process is simple and involves a lot of techniques you are already using. It has been around for many years and endured decades of abuse and the product is still top notch. After more than 30 years, the Stits process is still touted in the full scale aircraft market as the best. We believe modelers will accept it as a premium product and recognize the reputation already established.
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