Fiberglass Cloth Application

by: Dick Rihn


Disclaimer

There is nothing original about what I am writing below. I have talked to lots of people and have made many mistakes. In the long run, what I produced is satisfactory. I made many tests on one foot square pieces of plywood until I was satisfied with the technique and my application of the technique. These pieces were tested by attempting to remove the glass from the plywood. When I was satisfied with the results I covered my ailerons which are easier to do because of their smaller size. Then I was ready to tackle the wings. This can be done in any position I am sure. I chose to do it with the wing in horizontal inverted (inverted flight) position . Then I rotated the wing to the upright position with the trailing edge down to allow wrapping the cloth over the leading edge to allow working on the lower surface near the leading edge. I finished with the wing in horizontal flat (upright horizontal flight) position. All large orifices were covered over. The inspection holes and aileron push rod slots had been protected. The leading edge hole for the pitot tube was left unprotected. My final panel covering job took one hour from first mixing of primer to walking away to await a “cure” of the epoxy. It did not require an assistant, although that makes the job go smoother and quicker.

Purpose

Fiberglass cloth is bonded to the mahogany plywood surfaces of the One Design to provide a “filled” smooth finish surface that is bonded to the wood and will allow a subsequent bonding of the two part epoxy primer.

Materials

  • Sandpaper wet/dry 150 to 180 grit
  • High pressure air gun 80 -100 p.s.i.
  • 3” wide paint roller(s)
  • 3” wide ½” thick yellow foam roller applicator tubes (you may cut three from a 9”tube with an Exacto knife)
  • Wooden mixing sticks (tongue blades)
  • West System epoxy (105)
  • West System epoxy hardener(s) (206 &/or 207)
  • West System microlight filler (410)
  • Small disposable paint roller tray for 3” roller
  • Fiberglass cloth 1.45 oz. per sq. yard (also called “deck cloth”)
  • Protective gloves for the hands
  • Protective face mask to prevent inhaling sanded dust
  • Protective clothing or “old clothes” (the epoxy will get on you for sure)
  • Sharp scissors
  • 3” wide masking tape
  • Two plastic squeegees 6-8 inches wide with curved up ends
  • One metal wall paper hanger’s squeegee (or painters edge guide) 16-18 “ wide
  • One Exacto knife with fresh blade replacements

Preparation of the Surface

Before any further preparation recheck to be certain your pitot tube line is accessible from the wing root, seal all orifices of the wing to prevent dust entry. The use of cardboard and masking tape should allow you to seal the aileron push rod slots and the aileron bell crank inspection holes from within . The goal should be to provide a complete seal with no tape on surfaces to be painted.

Seal all nail holes from tack strip use. To fail to do this part of the procedure will compromise the cloth application as bubbles will develop which must then be laboriously trimmed and filled. The holes can be sealed by use of activated resin to which has been added 410 microlight filler mixed to thick almost paste like consistency. Seal each hole with a finger tip application under pressure. Yes, it’s laborious, but it’s less trouble than taking care of the blisters which will develop if you don’t seal these tiny holes Let this cure thoroughly before sanding smooth. Too early sanding only gums up the sandpaper.

Sanding should be done with a fairly coarse sandpaper 150-180 grit is fine. This leaves a rough surface to which the epoxy can bind. Do not attempt to produce a glass slick surface at this point. Now wipe and blow all of the dust off the wing. Let the air settle for awhile and do it again. This dust is fine and floats around, so cover everything you don’t want to have a fine patina of dust added to its surface.

Fiberglass Cloth Preparation

The cloth comes in a roll and is 50 inches wide. This will require a separate piece added to the wing root just as you had to add a separate piece of plywood at the inboard wing root. Unroll only what you will need for the length of the wing panel. Allow at least three or four inches overlap on both the inboard and the outboard ends as the cloth will tend to move during application and you want to have sufficient cloth to cover the span. Some trimming in the aileron region will allow greater ease of handling, but still allow three to five inches of excess. Overlaps do not have to be extensive as in ceconite/dope work. Remember the epoxy binds the fabric to the wood. When overlapping the leading edge you will want the top panel to extend unbroken to beyond the leading edge so the overlap will be hidden from view on the bottom of the wing. That means you may have some resin dripping down onto the lower wing panel. To prevent this, use a line of wide masking tape which is adherent only on the forward edge but hangs loose on the trailing edge side. This will provide a drip shield and direct excess epoxy to the floor. Taping the cloth in place, however, will lead to loss of freedom in applying the resin without wrinkling the cloth. Don’t waste your time trying to make the cloth go around sharp corners such as on the trailing edges. Just cover the flat surfaces. At a later time the flat surface of the trailing edge can be covered. Otherwise the slight stiffness of the cloth will lead to air bubbles at the (almost) right angle junction of surfaces.

Application of Resin

Put on gloves and respirator if you don’t want to inhale fumes (there aren’t many with the West System) and mix resin (5 to 1) with hardener. Only one or two ounces at a time will work just fine. Pour the resin into the small roller tray. Remember the smaller the amount of resin in a container and the thinner the film of resin the longer the pot life. It takes volume with limited surface for radiation to allow the heat build-up that triggers the exothermic reaction. My project was done with 206 hardener in 70 degree temperatures and it worked out just fine. As the area where you began is hardening you are working outward from the center with freshly mixed epoxy, so pot life becomes almost a non-issue. Begin in the center and work towards the periphery. Apply the epoxy with the 3” roller. With a little practice you will know how much to apply. Too little and it doesn’t penetrate. Too little and it allows the cloth to “float” and creates more wrinkles than you would like. Not enough and the wrinkles are difficult to work out. Run the roller outward from the center. Do not go back and forth as this will lift up the cloth on the “return” stroke. Wrinkles will develop more as you get to the periphery and over the leading edge radius.

Keep moving. Try to get the worst wrinkles out as you go but remember there will be more appearing as you finish. To work out wrinkles you can slide your gloved hand along applying friction moving at right angles to the longitudinal axis of the wrinkles. If near the edge of the cloth a slight tug on the edge of the cloth will help. If you work too hard at removing the wrinkles you will create wrinkles at right angles to the ones you just removed. Find the happy medium, which is easier said than done. For the flat surfaces aft of the spar a very useful tool is the metal edged squeegee. Be careful how you wield this as the corners can tear or deform the cloth. The plastic squeegees which have arced smooth ends are very useful but don’t have the power of the metal squeegee. The human hand is good in that you can feel wrinkles that are difficult to see. When you have a perfectly smooth surface over the entire piece continue to observe it while the glue cures. If bubbles, develop pop them. If wrinkles develop, straighten them out. It may require application of a little more epoxy sometimes to allow this to be done. Remove as much resin as you can by using the squeegees. This will keep the weight down and reduce the amount of finish sanding you have to do. Be careful using the squeegee on the edge of the cloth. It is easy to “fray” the cloth. This results in long streamers of fiberglass threads which require lots of work to sand out later.

Trimming , Finishing & Bubble Correction

Don’t be in too big a hurry to trim the cloth edges. Let two days elapse and they will trim easier. Just carefully run an Exacto knife down the edge holding the soft non-glued cloth taut and it will be a fun and easy experience. Sanding will finish the edge. Remove the masking tape and sand away any drips which shouldn’t have happened. If there are blisters wherein the glass cloth is not in contact with the wood it is quicker to cut the cloth away and sand the area lightly. Then blow the dust off and apply a surface of activated resin mixed with 410 filler. This will adhere to the wood and will also make a nice transition to the glass cloth edges so the whole thing will disappear when prime coat is applied. The color differences will look bad so judge your success by closing your eyes and running your hand over the “patched” surface. If it feels good, it is good.

Miscellaneous Additions

The wing root trailing and flat surfaces still need to be covered. Although these can be done while the large surfaces are still uncured it is a lot easier to do it stepwise one at a time. The process is the same as listed above. The pieces are small and very easy to work with. This will be fun ! A quarter inch overlap is adequate on the flat surface wing root areas. On the trailing edges aileron wells just trim to the sharp margins and then finish trim and sand when cured.

Finish Sanding

Now the fun part, finish sanding. Use Wet/dry 150 to 180 grit to remove the slick surface sheen. Do not sand through to the glass cloth. When done the opaque off white color should be uniform throughout and there should be no shiny surfaces By using the sand paper wet it doesn’t load up with gummy resin dust. A long sanding board will be helpful to do the job more quickly. However, remember this is a wood wing. Each day the humidity changes slightly and the tautness of the wing skin will also change. This will alter the flatness of the surface by one or two thousandths of an inch. Remember we are doing “finish sanding” here. So be careful with your long sanding block. You may need to resort to palm sized finish sanding. When wet sanding you will be pleased to see the reflections of objects on your wing surface. Hopefully the straight lines will reflect as straight lines on your wing surface!