Q-talk 35 - COMPOSITE TIPS
- Category: Q-Talk Articles
- Published: Monday, 31 August 1992 07:11
- Written by Mike Bergin
- Hits: 2319
Many issues ago a builder suggested that all the filling, blocking and sanding of the flying surfaces could be done before bonding them into the fuselage. Well I could just take that guy out for a good hardy meal for all the extra aggravation that he saved me with this tip.
I'm going to discuss the finish technique I used in preparation for paint. All of the steps, short of the final sand and paint, were done on sawhorses then bonded into the "hull" later. Two things are gained. First, and most obvious, the repeated flipping of the wing and canard to fill, block or sand and prime are much easier. Second, and not so obvious, is that the process of filling, blocking, priming and blocking/sanding MAY change the contour of the airfoil ever so slightly from the just laid-up surface. Soooo, if you made templates to fit the outside wing contours (to aid setting the angle of incidence and installation) before filling and sanding, these templates may not fit well. Check it. This could result in a change of maybe a half-degree. I understand from the newsletters that some of you boys have problems with wing and/or ground incidence. Food for thought ....!
OK, enough speculating. Here is one way of getting done. First, buy or build a pair of sturdy sawhorses if you do not have them already.
Next, fit the main and canard wings into the hull first. I did this before I even put on the slot cores. When in place, mark the hull to wing joint intersection line with something like a SANFORD laundry marker. Draw a line all the way around the chord of the wing or canard. More or less guess at the location of where the bottom of the fuselage will intersect the canard.
Then prepare the wings surfaces by sanding or grit blasting up to the pen marks. A word on this! Sheehan specifically said to me that one can sand the laminated surfaces down to where the fabric weave surface is sanded away to 75%. I'm not so crazy about this approach. I've said in previous articles that anything we can do to maintain structural integrity is a bonus. Therefore we remove some of the fiber if we sand the composite surface. I agree that some of it is the warp (or cross) fiber that holds the fill (or UNI) fibers in their place, but I'm of the conservative school of thought and tend not to want to remove any fiber.
I prefer to use small portable grit or sand blaster that one would find in a paint and body shop. Can be purchased at the nearby paint and body supplier. However, the best I've come across is from TIP in Ohio (mail order 800-321-9260). I use play sand from the local home and builders supply (84 Lumber, Lowe's, Scotty's, Moore's, etc). Use about 100 psi of pressure and go to it. It takes a bit longer because the blast pattern is only about 3/8 inch in diameter but my "tail" is worth it. I like the extra factor of safety! The grit blasting just etches the resin matrix and does miniscule damage to the fibers.
My project has always dragged out (no comments Masal) so the time between the last step and the time to finishing the wings was quite a while. If this happens to you as well and/or you have used the grit blast method or some air tool to sand your surfaces, they may be contaminated. Furthermore, if you have heated your workspace during the winter months with a kerosene heater you stand a chance of having contaminated surfaces. To check these just drop a drop of DISTILLED water on the surface and watch to see if it beads up or wets out. If you observe beading, your surface is contaminated.
Clean your surface with MEK solvent (purchased at the local ACE Hardware). DO NOT USE ACETONE as it evaporates too fast and won't do you any good. If you cannot find MEK, a "medium" grade of lacquer thinner is the second choice of blended solvents. Take the necessary safety precautions and use a good grade of glove to handle these materials. Butyl rubber gloves are best.
After washing the surface, prepare for the application of the syntactic foam. First apply mixed WEST System by means of something like a bondo squeegee then remove most of it with a clean rag. This will assure a good bond. The WEST epoxy is an aliphatic amine epoxy and by its chemical nature will bond better than the EPOLITE (SAF-T-POXY) which is an aromatic amine epoxy.
When working with the WEST epoxy you will soon find that one can't mix it with the micro-balloon as dry as you could with EPOLITE. The desirable consistency is that of peanut butter. Essentially what you look for is the point when the mixture is just thick enough to stand up into a peak when the stirring stick (tongue depressor) is stuck in and pulled out. This is perfect!
The best tool I've found for the application of dry micro is a piece of 1/16 inch G-10 fiberglass. Hello!? What is G-10? The electronic aces out there would know it as printed circuit board material without the thin metallic face. Some of the model airplane builders know it as the thin fiberglass sheet that is used for a variety of component mounting. Your local plastic supplier should have it. Those who can't get it can use something thin and flexible like sheet aluminum. I've made a variety of sizes but the one I use most is approximately 3 x 10 inches.
The most desirable WEST System combination is the slow drying system that has the designation of 105 resin/205 catalyst. The 206 catalyst dries too fast to be able to work with it. Don't get cocky on me now. Even though the label states a drying time of a couple of hours it is still too quick for some reason. There were times that I could only get a couple of passes with the squeegee and the stuff would start pulling back up and would not lay down.
Their 410 filler is head and shoulders above the 3M microballoon that we commonly use. The WEST System spheres are plastic coated balloons (or spheres) with a phenolic resin. The coating promotes better adhesion with the resin and therefore it "feathers" out nicely when sanded. Furthermore, this coating promotes an affinity between the dry spheres and they do not explode into a dust cloud with the slightest of sneeze.
From here on the steps are the same as with any dry micro application. Fill and sand! Keep in mind that you want to run the dry-micro only to within five inches or so of the marks made for the fuselage intersections. Remember that you need room for a good glass to glass bond of the fiberglass tapes and you will make wing fillets in this area later.
**Aeronautical Tip** This one is free and very hard to find! The fillet should be sized to have a radius having a length that is 45 to 48% the maximum thickness of the airfoil at the point where it meets the fuselage. This is found in Hoerner's Dynamic Drag book. If anyone wants a photocopy of the chart just write me.
One sanding tip that I may offer at this point. I used a homemade sanding spline board as when blocking out the dry micro. I found that the ones that you buy at the local auto paint and body suppliers aren't long enough to assure a flat surface. The dimensions of my board were sized to take a half width of sheet of sandpaper and three times the length. So this gave me a board 4 1/2 inches wide by 33 5/8-inches long. I made it from a piece of 1/2-inch plywood. I added a two wide 1/2-inch solid pine backbone that ran full length on edge, screwed and glued to assure flatness and provided a handle. The bottom surface was covered with duct tape to provide some cushion and a surface that permits continual reuse of double stick carpet tape to hold the sandpaper.
I use 40 grit paper to do most of my blocking after the dry micro filling. Then I come back with 80 grit to get most of the deep sanding scratches. An old carpenters pencil trick can help you perfectionists to achieve a very flat surface while blocking. Simply scribble on those suspected low spots using the side of the pencil lead. As you block of course the remaining pencil scribble will indicate the low spots but won't contaminate the surface for the next coat of dry micro. You simply won't get them all but that is where the primer comes in.
When all the surfaces are where you want them you are ready for primer. The primer that I recommend is the PPG Ditzler DP-40 epoxy primer as a sealer/bond coat and the acrylic urethane K-36 PRIMA for the filling and sanding. The DP-40 is shot with a tack coat and one wet coat. Mix the one to one ratio and do not thin. Wait at least fifteen minutes before shooting on the K-36. Mix the K-36 at a ratio of five parts primer, one part catalyst and one part thinner. Lay on kind-of-a-tack coat for the K-36 and then two to three good wet coats. Let this dry for twenty-four hours before sanding. This amount of time allows the primers to fully cure and work down into the 40 grit sanding scratches that will be left.
Use your long spline board that you have made and block the K-36 primer with 80 grit paper sanding most of it off. Don't worry if you have sanded into the epoxy sealer coat. Just don't go overboard. At this time you will see the low spots that are remaining. You may use polyester or lacquer spot glaze putty or even a little Bondo. I recommend one of the micro-balloon filled body putties. If you look at the can it only weighs seven to seven and a half pounds opposed to twelve pounds. This stuff will be more flexible than the lacquer or the polyester spot glaze.
Work all your spot glaze with 80-grit paper because it cuts nicer and you avoid introducing local high spots in these glazed areas. When satisfied, give it the next three coats of K-36 primer. You may bond your wings at this point or give it another block sanding with 180 grit and prime. At this point all you would have to do is work the local areas around the bond joints and fillets, sand with 380 grit WET-R-DRY paper (dry sand) and you are ready for paint. On the other hand you may want to hold off on this last step if you feel that you are going to contact a lot of hangar rash before you paint. Then work the whole airplane with the 180 grit and the last primer coat.
I may be contacted directly for any answers or information at 410-827-9386. If I'm not there, leave a message on my answering machine.
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