In our ongoing series on covering using nitrate and butyrate dopes, our next step is to actually spray on the dopes. When you are covering an airplane with fabric, most of the work is done when you finally get to the spraying stage. You will already have all of the fabric in place, the rib-lacing completed, all of the tapes, drain grommets, etc. will have been installed, so you are ready for the fun—the actual spraying itself.
Just as a quick review, ensure that you have the proper equipment, a good place to spray, and that you choose the right day to begin the process. Unless you have a climate controlled spray booth—doubtful—then the perfect spraying day would have a temperature of about 75 degrees with relative humidity of 30% or less. We all know these days are hard to find. With that in mind, do not spray butyrate dope in temperatures below 65 degrees or above 85 degrees. Also the relative humidity should be less than 50% if at all possible but 70% is acceptable.
Do not spray in direct sunlight or in the wind. Try to make up a small spray booth as we discussed in the last article. It will save you from a lot of problems. Don’t forget to wear the respirator and to use the spraying techniques we discussed.
SPRAYING BUTYRATE DOPE
After all of the tapes, etc. are in place, you will begin to spray coats of non-tautening butyrate dope. Before you begin to spray heat up your small iron to 225 degrees F and carefully iron down all pinked edges of finishing tapes. This will ensure a nice, smooth surface.
You will want to spray on 3 cross coats (a cross coat is one pass with the spray gun north and south followed by another pass east and west) of non-tautening butyrate dope. The idea is to achieve a smooth, plastic finish. You will need to thin the butyrate dope with butyrate thinner. The ratio should be approximately 1 part of thinner to 1 part of butyrate dope. If the relative humidity is high (above 50%) and/or the temperature is high, you may have to add a retarder. This mixture will usually be about 1 part retarder to 3 parts of reducer. Let’s take a look at what a retarder actually does and why you may need it.
As dope dries, the rapidly evaporating solvents actually lower the temperature at the surface of the object being sprayed. Any water vapor in the surrounding air will then condense on the surface. If the humidity is 80% or more this condensed water vapor will give the dope a milky appearance. In the fabric covering world this is known as “blushing”. Any blushing that occurs with dope will actually weaken the dope itself and its bond. All blushed dope must be sanded off and the area resprayed. With this in mind, you can easily determine that you do not want blushing to happen. Remember, this will usually only occur when you are spraying in a humid environment. You have two choices: wait until a better day or use the chemical known as “retarder”.
Blush retarder is a special solvent that will slow the drying process of the dope, thereby minimizing the chance of blush. It is mixed in with the butyrate dope and thinner prior to spraying. Retarders will not solve all of your problems. If you elect to spray on a 95-degree day with the humidity at 99% retarders will not prevent blushing. Wait for a good day. Another tip, do not wet the floor of the area where you are spraying just prior to beginning the application. If you do this the humidity will increase. If you need to wash the floor let it thoroughly dry before you begin to spray. Finally, use only the recommended amount of retarder. Use of excessive retarder can cause dope separation.
Lets return to our spraying process. Prior to spraying on the first coat of thinned butyrate, thoroughly clean the surface using butyrate thinner and a clean rag. Use only cotton type rags. Do not use any rag that may have any oils or other contaminants. After this step, just prior to spraying wipe the area with a commercial tack cloth. This will remove any lint, dust, etc.
First Coat of Butyrate
Spray on a full cross coat (2 coats) of thinned, non-tautening butyrate dope. If you need to use a blush retarder add it as you are thinning the mixture. Completely mix the solution and then pour it through a paint filter into your spray gun. Spray the mixture as we discussed in the last issue. Spraying dope is fairly easy due to its consistency. It is somewhat thick so it will not have a tendency to run or sag unless you use the wrong techniques. After you have sprayed on the first coat make another pass with the spray gun in the other direction to complete your cross coat. Remember to always begin this process on a small surface such as a control surface. This will allow you to practice and not make a mistake on something large.
Second Coat of Butyrate
After the first coat of butyrate dope has dried for at least 1 hour (this will depend upon temperature), spray on another coat of butyrate dope. Again, allow this coat to dry for at least 1 hour and then apply a third coat.
You will now have 3 full cross coats of butyrate dope on the surface. The fabric should have a smooth plastic appearance.
After applying the third coat of dope, it is time for our favorite activity, sanding. You should use 320 grit wet or dry sandpaper and thoroughly wet sand the entire surface. Do not sand over rib laces, rivet heads, or over anything that might cut through the fabric. Use of a sanding block is very helpful. As you wet sand use a hose to wash off the residue. Do not leave any residue on the surface, as it will prevent additional coats from bonding properly. The area you have sanded should feel very smooth as you run your hand over it. You will be able to detect areas that need more attention with experience.
After you have sanded the entire surface, set the part aside and let it dry completely before you spray on any additional coats.
Some people will apply additional coats of butyrate dope for a smoother finish. It is not necessary to apply more than 3 cross coats. However, if you desire a really smooth surface you may apply 2-3 more coats after sanding.
Silver Butyrate Coats
Polyester fabric, like most fabrics, will deteriorate in sunlight if not properly protected. This deterioration will occur within a very short time—6 months or less, without protection. All types of fabric will deteriorate in direct sunlight unless protected. Polyester fabric is not as susceptible to this problem as cotton; however, if bare polyester fabric is left in direct sunlight for 12 months it will lose over 85% of its strength. Cotton fabric exposed to the same sunlight for the same period of time will deteriorate almost completely. Polyester fabric is protected from the UV rays of the sun by applying chemical coatings containing aluminum pigment. Application of the recommended number of aluminum coats will provide adequate protection for years. These coats are termed “silver coats” even though the actual mixture itself consists of an aluminum pigment mixed with butyrate dope.
Randolph markets a pre-mixed butyrate dope that contains the proper amount of aluminum pigment. The aluminum pigment in these premixed chemicals has a tendency to settle to the bottom of the container. It is absolutely imperative that you mix the pigments into the solution just prior to spraying. Failure to do so will leave a mixture that will not adequately protect the fabric. Often a 5-gallon container of either of these chemicals will have collected most of the pigment in the bottom just from settling. You will find the pigment has settled to the bottom of the can to the extent that it feels like a hard layer. You must use a stirring stick or some means to break that loose and mix it throughout the can. Again, I want to emphasize that this must be done just prior to spraying. The aluminum pigment is heavy and will settle to the bottom.
If you desire, you can purchase aluminum powder in a can and mix it according to directions into butyrate dope thus making your own mixture for the silver coats.
The silver butyrate dope should be thinned the same as our clear coats—one part of thinner to one part of silver dope. If you need to retard the mixture do so the same as previously discussed. Silver dope is also subject to blushing.
You should apply at least 3 cross coats of silver onto the surface to provide adequate protection from the UV rays of the sun. A quick check to ensure the proper amount is in place can be done by placing a 60 watt light bulb inside the structure and make sure no light is visible on the outside. (Use a protected bulb that will not shatter and ignite the solvents inside the area.)
Wet sand after each coat of silver. Allow the surface to completely dry before you begin to wet sand. After sanding allow the water to dry before applying the next coat. Always clean the surface and use a tack cloth just before spraying on a coat.
Additional coats of silver may be applied for a smoother finish. Sand after every other coat using progressively less coarse sandpaper. In other words, begin with 320 and then go to 400 or more on succeeding sandings.
This last step of spraying on the silver coats brings us to the final color coats. This will be discussed in a later article.