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Hoerner Wingtips Tanks for Sonerai II – Part 3

From Bits & Pieces Newsletter, March 2015 Issue

By Bill Evans, EAA Chapter 266, EAA 794228

Last month we discussed making the moulds for our new wingtips. This month we describe how to create the actual wingtips.

1. Layup: resin cloth and layers. With the male plugs made and prepped for laying up fibreglass cloth and epoxy resin, this part begins.

We placed the plug with what will become the outboard side facing up. One wood template remains attached, which provides 3/4-inch space for wingtip attachment. Newsprint was used to protect the table the plug sits on.

These wingtips will be holding 100LL avgas so that methanol or ethanol degradation may not matter, but someday we will only be able to get 91 premium auto gas or worse. That day it may matter what the fuel contacts, hence epoxy resin. In our case, epoxy resin was mixed with micro balloons to create “gel coat”.

What you need:          

  • 8-ounce fibreglass cloth – 100 square feet or 9 square metres
  • 24-ounce (approximately) fibreglass cloth – 30 square feet or 3 metres
  • 4 litres of epoxy resin – 2 litres with the 45-minute hardener, 2 litres with the 20-minute hardener.
  • 1 litre of micro balloons
  • acetone – 1 quart
  • goggles, dust mask, and box of latex gloves
  • scissors, pinking shears, needle and thread, masking tape, Sharpies, 1-inch brushes, and spreaders
  • shop towels, a dozen pudding cups, and roll of plastic wrap
  • work lights on stands or bright halogen lights
  • If you know the vacuum bagging technique, get your vacuum pump out.

Mould vacuum bagged

We decided on the day of the layup to use some shrink-wrap for wrapping the plug from end to end to provide better ways of getting the layup off the plug.

We needed two layers of the 8-ounce cloth and one layer of the 24-ounce cloth for each wingtip shell. In this case, the cloth was laid out over the plug (top and bottom sides) and cut to shape with the pinking shears. Allow a 1-inch overlap and 1-inch edge overhang. The overlaps were pinned, then sewn on a household sewing machine. (Diversified skills, people, think diversified.) Lacking a machine, you could sew by hand. Invert the three fibreglass socks, fold carefully, and set them where they are handy. Three fibreglass socks will make one shell. Snug is the operative word for the socks. Note that the seams of the socks should not overlap each other, or the wingtip will have bulges.

Take the layup operation outside if you can. Otherwise turn on the ventilation. While the epoxy has little smell, we would not do this in the basement if your family is home. Use the garage or an outside building. A litre of curing epoxy will cause complaints, at least.

We used Miapoxy 100 (Freeman) with the 45-minute and 20-minute catalysts for the epoxy resin. For the gel coat, the 20-minute hardener was used, 45-minute pot life hardener for the layup itself.

Gel coat. After mixing about 1/8 cup of resin in a ratio of 4-to-1 resin to the 20-minute hardener, we added two heaping tablespoons of micro balloons (also Freeman) at a time until I had a white resin that could be brushed onto the plug without running much. This white resin is our equivalent of gel coat. If needed, you could do this twice. The idea is to prevent the fuel from contacting the cloth. The “gel coat” also provides a smoother surface inside the tank.

Once the gel coat has become tacky, mix 1/8 cup of 45-minute resin, ratio of 4-to-1. Brush lightly onto the gel coat. Take one of the 8-ounce socks, lay it on the correct side of the plug, and unfold it carefully so that it contacts the plug evenly. Work folds and wrinkles out now. We like to use brushes; many builders use only spreaders. Work in the resin to eliminate air bubbles and gaps.

Once one fibreglass sock is wet, lay on and unfold the 24-ounce sock in the same way. You will need to keep mixing resin. Stay with 1/8 cup at a time. That’s one reason why you use the longer pot life. Work in the resin until the heavy cloth is wet. Lay on the third sock, 8-ounce cloth. Try to avoid having them overlap one on top of another. By now there will be a lot of resin in your layup. Work to squeeze the resin up from the wet layup into your top layer of cloth. Use a plastic spreader for this – carefully. Some builders wrap the whole thing in plastic (Saran) wrap at this point to protect the cloth from the spreader. We tried this.

Mould cling-wrapped

Being careful to avoid folds and air gaps; use the roll of plastic wrap to wrap the whole layup again. Once wrapped up, the idea is to hand-squeeze the layup everywhere to get it as tight to the plug as possible.

If you have a vacuum pump, it may be employed with polyester tricot under the plastic wrap. This is called the rip-ply method, taught at Sun ’n Fun workshops. Make a slit in the open end of your layup to insert a rubber hose covered in paper towel. Seal it up and switch on. You will need to work the edges of the plastic wrap to get a seal. The vacuum will rise when you have a seal. We left the pump on for about four hours.

If you have bright lights, place them 15 to 18 inches from the layup and turn them on. You can’t just walk away or go out and leave this layup curing. It needs to be watched. We found that after 2 to 4 hours the vacuum has done its work and may be switched off. A few hours after that, the plastic wrap may be carefully removed, before the layup gets really hard. After 8 to 12 hours you can rip off the tricot rip-ply if you used one. The vacuum bagging often employs a tricot rip-ply layer. (See above.) The rip-ply gives you a very nice and smooth exterior finish. Carefully done, it might be mistaken for a female mould.

The layup may require a longer time in the lamps to become tack free. Don’t leave the lamps on overnight. Use your hands to see how warm your layup is. If it’s uncomfortable to touch, it’s too warm. Move the lights back. If you must leave or sleep, turn the heat lamps off.

Try to trim the edges of the layup (at the plywood template) before the resin is fully hard. Shears or tin snips worked well enough for us.

2. Removing the plug. Many builders have compressors which they employ to pop the layup off the plug. If you will be doing this, make 1- to 2-degree angles in the ends and sides of your plug to ease removal. We did not because we needed the fuel capacity. Two-inch-wide strips of tempered Masonite, plastic corrugated sheet designed for signs, or similar material can be used with a rubber mallet to separate the layup from the plug. On the first layup, we removed it carefully from the plug to save some of the foam for bulkheads and fuel baffles. In any event, it took half an hour or more to get the layup off without damage. Be careful not to crack the layup during removal.

The second plug was not needed again, so it was simply cut and pried apart to get it out of the layup quickly. You need to preserve one of the plywood templates for sizing the fuel bulkheads.

We removed both of the layups at the same time but only fibreglassed the fuel tank in one that week. In the meantime, the second layup distorted. The top surface became a little convex. Therefore, we suggest you keep the second layup on the plug until the bulkheads are glassed in.

3. Sanding, filling, priming, and painting the wingtip. You need a shop coat, gloves, mask, and good goggles for these steps. The masks with the plastic valves by 3M largely prevent the goggles from steaming up as you work. They are much cheaper from safety supply stores by the box, but they are not cheap.

If your layup is perfect and the cloth joints are edge sewn, your layup may require just light sanding before filling primer and paint. Our plug was not perfect. We carved the block on the horizontal, and all the seams that were doubled at the leading edge to the layup needed to be sanded to uniform thickness and smoothness, which was a lot of work.

We needed to do a lot of sanding and several layers of filling with the best body filler. Bondo would not serve here. It’s just as well we had to fill, because that enables us to write about this. We spent more on filler and discs because the plug was not perfect.

Materials and equipment. This may cost $100.

  • Evercoat Spot-Lite Premium filler
  • pack of spreaders
  • plastic board (chopping board) to mix the Evercoat above
  • one roll of (100) 6-inch No. 80 grit sanding disks. These have adhesive; some use Velcro. Adhesive is preferred.
  • one 6-inch rotary air sander; use electric if you have it. The two we used have safety covers.

Rotary sander

  • two to three No. 40 grit sanding discs
  • roll of shop towels
  • spatula to clean filler from your board and spreaders
  • steel wire brush

If you don’t have a good compressor, you could run two small compressors into a steel pipe (pig) and use them together. Failing all of that, you could get an electric rotary sander (gasp). Of the electric brands sold in the stores, possibly Cincinnati makes the best. In any event, study them to see which have the best safety covers and which deliver the most power. Horsepower counts because of the way sanding is accomplished.

Ideally you want 100 pounds per square inch (psi) air pressure and 10 cfm or more for compressed air rotary sanding. Even so, you will need to plan your work to let the compressor catch up.

Technique. The Evercoat is mixed in quantities of about a golf ball in size with a line of blue catalyst cream an inch long. It is scooped onto the mixing board with the spreader and the catalyst mixed in with the spreader. You press the catalyst into the filler to mix it and scoop up the edges and press the filler back down into the center. Repeat this until the filler is evenly mixed and a very light blue colour.

Get the filler on the layup! It cures fast on the mixing board. Spread it on evenly to cover the surface as far as the mixed quantity allows. We’ve been spreading it 1/16-inch thick.

The pot life of the filler may be 5 minutes. So get it spread and leave it alone. In practice, the filler needs to be a little bit thicker than needed so it can be sanded smooth. We did not make our plugs to a perfect shape so two cans of the Evercoat were consumed. It’s a nice product to use and sand, but we found it expensive.

You do not want to wait until the filler is fully hard before sanding. After maybe 5 minutes from the time the filler starts to set up, it will be possible to drag a fingernail across the filler without leaving a mark. Keep trying it. That is the point at which sanding starts.

The rotary sander needs lots of cfm partly because you are not sanding with just one edge of the disc. Not at all. You place the whole sanding disc flat on the surface and seek to remove the excess filler as efficiently as your sander and compressor allow. Once a disc ceases to cut efficiently, replace it. Each minute the filler is getting harder. This is why you have 100 discs.

Once the filled surface is sanded smooth and even, stop, evaluate where to fill next, and repeat the filling and sanding process. Frequently use your good work lights to look down the layups to get them straight in X-Y axes and level. We found that three coats of filler are minimal to fill both the texture of the layup and to get it straight.

This is not just mindless filling and sanding. Study the straightness of the surface often and work to get it as close to perfect as you may. Every time you walk around your aircraft you will see the imperfections and hate them. More plain-minded builders also may inwardly cuss. So do your work well. It not only matters aerodynamically; it also shows.

Filled and sanded wingtip

Now I am privileged in our chapter to have helpful members who have worked in the auto body field for 70 years and more. After that long, you get to be good. Ross Holden is one of them, and he can do by rote what I have to struggle to do. And Ross has professional everything for the job. You should see his antique tractor. I digress. Thanks, Ross!

Prime and paint. Once the filling was done, I drove straight to Karl Heindl’s for priming and paint. Karl also has made this case of painting as a science. He has the best filling primer in hand. He gets it in spray cans and with a handle, is able to lay it on so that it fills, and does so without runs. This stuff dries to the touch in about 10 minutes. Again, lights were used. One secret for painting is that the wingtips are placed on sticks extending inside the wingtips and beyond the end of a table. Thus they can be moved and turned as needed.

Sticks help to manoeuvre the part.

Karl brought these sanding sheets over from the UK. This sanding paper or cloth has been bonded to sheets of flexible insulating plastic. It’s a bit like plastic Permatex.

Well…it’s almost magic. You can take a full sheet in your hand and sand away to your heart’s content. It removes irregularities from the primer surface. It protects your hand. It doesn’t allow the sandpaper to tear. It lasts almost forever. It can be shaken like a rag to shed dust. I sanded three or four layers on each wingtip with one sheet of this stuff. It lasts forever. I’ve never seen it over here.

If you have ever seen the finish on Karl Heindl’s Europa XS…well, now you’ll have some idea how he eliminates all the blemishes.

These sheets are called sanding sponges. They can be purchased from Isaac Lord at Search for “Sia” on that site. The home page tells you how to order online.

One of Karl’s strengths is that he is fairly masterful at finding the best products on the Internet and then getting them at the lowest possible prices. However, we went to the local auto supply store and bought a can of the acrylic colour coat in a shade of blue that is almost the same shade as the blue trim on my Sonerai II. It might have been the same colour when the original paint was new. The colour? Why, it’s Sonerai Blue.

Pay attention because it was at this point I learned to spray-paint. Now, I have been spray-painting since 1962 when I painted my 1940 Chevy. Purists will complain that here in Montreal the Christofarro Brothers paint, but most everyone else does something far less worthy. We might have graduated a bit this week.

Once the wingtips and primer are filled and sanded, we brought out our large can of acrylic blue. And I started to lay it onto the wingtips. It was about this time that Karl, who is not shy, said, “Bill, you can’t paint.” He took the can, moved the lights just so, and explained while he painted.

Karl Heindl showed me how to spray-paint in about 5 minutes. No runs and no sags. You need those halogen lights or equivalent on a stand. Bright is the operative word here. You operate your spray gun/can, about, a foot from the wingtip. It is tilted at around 45 degrees sideways. You place the wingtip vertically and position yourself to be able to see in the paint reflection whether the paint line is continuously wet. The lights are at one end and your eyes are looking into the reflection at the other end. You are looking at the paint line.

Apply the top coat first with a fine thin dusting spray to start with, then 10 minutes later apply the thinnest coat you can – like this: Apply paint in a horizontal line such that there is just enough paint sprayed to make that paint wet. There are no speckled spots. Now apply paint adjacent to that first line, paying constant attention to getting a surface that is just barely but continuously wet – no speckled spots. Look at the surface for wetness. Keep your eyes where they can see that the paint being applied is wet and shiny on the surface or not. Judicious application is the operative term here.

Once the entire sprayed surface is “wet”, you can either wait or turn the wingtip enough to allow the next area to be sprayed. The whole area and the paint line must be just wet. And so on, around the wingtip you may go.

Once a line of paint is all wet, resist all temptations to spray on more. You should have achieved a spray paint application free of sags and runs. Don’t ruin it now by adding more. When done, study the wingtip in your work lights so you can see the light reflection in the paint; that is, see that the painted surface is wet and blemish free. Very carefully touch up if you must.

Both done and looking great

In the same way, it might be good to apply clear coat at least to the leading edge, since wind erosion occurs there.

Read earlier parts:

The installments of this series to come in future issues:

  • Part 4 – Compartments, fuel, strobe lights, and balance weights
  • Part 5 – End bulkheads and fuel baffle
  • Part 6 – Refueling adapter (nut ring)
  • Part 7 – Installation and hookup. 
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