Article - Spraying Chemicals Part One
Over the last several months I have presented the basic steps of fabric covering using nitrate and butyrate dope over Ceconite fabric. The steps involved in covering an airplane are essentially the same for all of the different systems available until you are ready to apply the chemicals. Differences do exist between systems but they are somewhat minor until reaching this stage. I will review the steps we have discussed that must occur prior to spraying the chemical coats and then we will begin applying the chemicals.
To begin with, the surface you are covering must be properly prepared. This is accomplished using an epoxy primer or an epoxy varnish following a thorough cleaning. After preparing the surface it should then be carefully inspected for any defects. Remember that you will probably not have access to the inside of the surface for a number of years. After the inspection it is time to select the proper weight of fabric and then attach it to the structure. Attaching the fabric is completed using special cement designed for use on fabric.
Once the fabric is secured in place on any component part, it is then heat shrunk using a regular household iron. The iron is calibrated to the required temperature and then the fabric is ironed until all areas have received the proper heat. This ensures that the fibers are adequately shrunk. The next step is to seal the fabric by brushing on a coat of nitrate dope or Rand-O-Proof. This encapsulates all of the fibers and provides the necessary adhesion for all subsequent coats. This is a very important step because the bond of this coat is essential for the remaining coats to properly adhere.
Next we attach the fabric to the wings and to all surfaces that provide lift. This important step is necessary to preclude the fabric "ballooning up" during flight due to the low pressure created on the top of the wing as a result of lift. We then cement in place inspection rings and drain grommets. Finally, several areas are covered using tapes that are precut from the fabric into various widths. The most common width of tape used is 2 inches wide. This last step brings us to the spraying of chemicals.
The first step after the entire taping, etc. has been completed, is to spray on additional coats of clear butyrate dope. Use non-tautening butyrate for this step. The objective is to spray on enough coats until the surface has a smooth, plastic like finish. After this, you will spray on several coats of butyrate dope mixed with aluminum pigment. This will protect the fabric from the harmful rays of the sun.
Before we start spraying these coats of butyrate dope, lets review some of the basics.
THE ART OF SPRAYING
Many individuals are unsure as to whether or not they have the ability to spray paints and dopes on their airplane. Let me assure you that you can learn very easily. It is not difficult to spray. This is particularly true when spraying the fabric covering chemicals. They are more viscous and have fewer tendencies to run than regular enamels. This way you can learn the spraying techniques with a chemical that is easy to spray before you get to the color coats. Don't be afraid to spray your own surfaces. With practice you will become very proficient. The word practice should be emphasized. Before you begin to spray on your airplane surfaces, find some old pieces of cardboard and practice spraying on them. Practice until you feel somewhat confident and then begin with a tail surface or a small piece of the airplane. Do not start with a wing or fuselage. Learn on a small surface so that if you make mistakes you can more easily correct them.
Lets discuss a few of the basics of spraying.
The first step in spraying is to select the proper spray outfit. Perhaps you already have a spray gun and compressor. If so, be sure it is a high-quality gun. Do not begin the spraying process with a cheap gun. Not using proper equipment will cause you untold grief as you spray. If you do not have a spray rig I would suggest you consider purchasing a High-Volume-Low-Pressure setup. Several of them are available such as the one shown in Figure 1. These systems are easy to use, especially if you have little or no spraying experience. They also plug right in to a 110-volt outlet and come complete with everything you need. In addition, they will save you money by using fewer chemicals. This is a result of very little overspray being created by this type of spraying outfit. In other words, the chemicals go on the surface and not into the air.
Where to Spray
Where to spray is the next question. A clean, well-ventilated area is necessary. Do not spray outside in direct sunlight. First of all, you do not want to use your basement for painting. The fumes will permeate the entire house and you will not be popular with your family. A garage or workshop can be more than adequate. After you have found a suitable location for painting the next step is to build your own paint booth. This can be done very easily by using PVC pipe and plastic sheets. Build a square frame out of wood or PVC pipe large enough to cover your airplane or the largest surface you will be spraying. You should allow enough space to be able to walk around the surface. You can hang the frame from your ceiling with pulleys so you can raise and lower it. Cover the roof and sides with plastic sheeting (4 mil painters plastic will work) stapled or taped to the frame. Tape the sheets together using duct tape. At one end of the booth place a furnace filter and at the other end an exhaust fan. This will provide filtered air. Make sure the fan has an enclosed motor with no chance of sparking. An explosion proof fan is best. If you are unsure about the fan, leave it out and quit spraying when the booth becomes full of overspray. The overspray will settle in minutes and then you can go back to work.
Temperature and Humidity
Under ideal conditions you would spray with a temperature of 75 degrees F and a humidity of less than 30%. Of course, in the real world this is not always possible.
When applying the butyrate dope, be sure the temperature and humidity are at least favorable. You can't always find the perfect environment but do the best you can. Do not apply the dope if the temperature is less than 65 degrees F or higher than 85 degrees F. The humidity should be less than 50% if at all possible. Remember that butyrate dope is highly flammable. Do not use space heaters or any type of open flame to heat a workshop area. Store the butyrate dope in a safe place. Don't use electric drills to mix the dope and thinners.
Certain health hazards do exist with spray painting. Of course, the hazards depend entirely upon the chemical that is being sprayed. The most significant health hazard occurs when atomized chemical particles are inhaled. You must protect yourself with an adequate respirator. A charcoal filtered respirator, such as the one pictured, is sufficient for most primers, dopes, and paints. Whenever you get to the final color coats you may need additional protection. If you are using any type of polyurethane paint you must have a forced air breathing system. Polyurethane paints emit polyisocyanides that can be extremely hazardous to certain individuals. Some people have severe reactions to polyurethanes so don't take a chance. A simple forced air breathing system is pictured and is available from Axis Products. It is relatively inexpensive and is certainly a good investment to protect your health.
You also need to protect your skin. Wear Invisible Gloves barrier cream or latex gloves when mixing or spraying. If you spill solvents on yourself, remove your clothes and wash the area with soap and water and put on fresh clothes.
Do not mix dope with an electric drill. The motor could spark and cause a fire. Fumes result from the stirring action that will rise up to the arcing drill motor. If conditions are just right a flash fire can ignite in the dope can.
Use eye protection in the form of goggles when mixing and spraying. Keep an eye wash station nearby in case of emergency. You should also have fire extinguishers handy that are rated for petroleum fires. Under certain atmospheric conditions the action of sanding or spraying can generate static electricity. When this static charge is transferred to a surface the resulting spark could ignite solvent vapors. Ground the structures being sanded or sprayed. It is also a good idea to wear leather shoes when spraying and sanding. This will ensure that you are grounded preventing any static discharges.
Cross Coat Method
When you spray be sure you use the cross coat method. Apply one coat north and south and then another coat east and west -- this is one cross coat. During our discussions I will continue to refer to a coat of material meaning 1 cross coat.
Your spray gun should be properly adjusted each time you spray. The actual adjustment of a spray gun depends upon the equipment you are using. The manufacturer should provide you with a set of instructions on setting up the gun. You should use the type of spray gun nozzle recommended for the type of dope you will be spraying. A test pattern should always be sprayed on a piece of cardboard before beginning to paint. The normal pattern for a spray gun will be fan shaped. To begin the actual application procedure, hold the spray gun approximately 8 inches from the surface you will be painting. (Spreading your thumb and index finger apart as far as possible can approximate this distance). This distance may vary somewhat depending upon whether you are using a HVLP system or a pressure spray gun. The spray gun should be far enough away so the paint does not run or sag when applied and close enough to lay on a wet coat. To prevent the paint from being uneven, it is imperative that the gun be held exactly perpendicular to the surface. If it is tilted the dope will be heavier on one side and lighter on the other. The spray gun should then be moved parallel to the surface only the distance you can comfortably move your entire arm while keeping the movement exactly parallel. If the gun is moved in an arc the material will be applied heavier in some places and lighter in others. You should squeeze the trigger of the gun just prior to beginning the paint stroke and release it just before it is completed. You then should move up or down approximately 1/2 fan width and begin the next pass. You must overlap the passes to achieve an even build-up. Each pass of the gun will usually apply the paint more thick in the middle with a tapering off on each end. Remember our definition of a cross-coat, one pass north and south followed by a pass east and west.
Of course, you will be spraying one part of the airplane at a time. You will want to spray the entire airplane up through the color coats while it is disassembled. Don't rush the job. Allow the parts to adequately dry between coats. (More on this later.) You will need to gather all of your materials, tools, and supplies prior to beginning. That means stirring sticks, paint strainers, thinners, tack cloths, rags, etc. Spraying is about 90% preparation and 10% actual spraying.
Next month we will discuss the actual coats of butyrate dope that will be sprayed on to complete our covering process.