Filler??

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Yellowhammer

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Feb 21, 2020
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62
I am at the proof of concept modeling phase of my project, and on our model, we have exclusively used EcoPoxy products with excellent results, to include filling & finishing on evaluation layups. It seems to be compatible with a prepared surface that was laminated with AeroPoxy products, so I can't vouch for compatibility with West.

The matrix is very easy to work with, has very little smell and is completely non-toxic. It can fully cure in temps as low as 55 degrees F, but I have not personally tried this; we do operations with the shop at around 70F.

Despite great results in load beam testing and such, we plan to use AeroPoxy on our full scale build, as the EcoPoxy folks are unwilling/unable to share mechanical properties data with us in order to validate our findings.

That being said, perhaps EcoPoxy is worth a shot for you. Best of luck!!
I like AeroPoxy. It does a great job in all facets of application. The Structural Aeropoxy is tough to stir but is very strong also.
 

Victor Bravo

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So how do the racers keep a perfect contour
on there planes, sand between races like the
sailplane racers did?do?
I only had a composite race wing for one year (1989) and to be completely honest I had a hard enough time keeping the airplane together, and not getting myself killed. So although I did sand and paint the composite wing when I first got it (from Grove Aircraft), I never had the experience of sanding it between races, or re-doing it every year. I am certain that the really really serious racers (Endeavor, et al) would likely be sanding their composite wings between race seasons.

I do seem to remember Alan Preston having the wings of the unlimited Mustang Dago Red fully filled, sanded and "sailplane contoured", and that in that process they found the actual airfoil did not match the design-specified NACA P-51 section. For more details on that, you'd have to talk to someone like Dave Lednicer (likely involved in that airfoil project).

I did do a little more of the sanding and buffing previously, in sailplane racing. I believe it was a lot more important in sailplane racing than in powerplane racing. The drag of the wing was a much much larger percentage of the overall drag on a sailplane. In the powered racers, we had many more sources of drag to think about and more "low hanging fruit", like wheels, landing gears, cooling drag, more large intersections, etc.

Mind you, the advances in manufacturing and the lessons learned in the design of composite structures have likely made this a smaller problem. Hand laid, wet laminations, with big globs of resin pooled in certain parts of the airfoils... are a thing of the past. It was the resin shrinking over time that caused the 1970's through 1980's sailplane racers to constantly be filling, sanding, etc. over and over. Bagging, infusion, prepregs, and more care in the layup design has probablby made large advances in holding contours.

On that subject, people like HBA's own BoKu and Billski will have far more definitive knowledge than I.
 
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speedracer

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Feb 4, 2020
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13
Here's a comparison of different filler weights I did. Superfill (A/Spruce) is a two part goop with the bubbles, epoxy and hardener already in it. It's heavier but way fewer pinholes and craters. I normally use West 105/206 (slow) hardener and Microlight 410 because it's lighter and sands a lot faster than the others. On wood for guaranteed adhesion I'd paint the surface first with straight West 105 and 206 (slow) hardener then squeegee and wipe off as much as you can with paper towels. Then apply micro (with 206) before cure. After fairing in there will be several thousand pinholes to deal with. Instead of chasing them down and filling them for days (all the ones you missed will show up when you start spraying primer) do a 4-5 coat epoxy "wipe" with West 105/205 (fast) hardener all in the same day. Paint or squeegee a coat on then squeegee off as much as you can. Wait till it gets tacky and repeat. ALL the pinholes will be gone forever. Then, of course, another round of sanding. I sand through 150 grit then prime with Awlgrip epoxy primer. Then...… another round of sanding through 400 grit, then paint.
 

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dog

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Dec 29, 2019
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Ok that gives me a bit mote background on the why of sanding and re sanding in composite wings.

I started to speculate that in wood wing construction,if the outer layer of finish was "decoupled" from the underlying structure with multiple (thin) layers of something that would flex and stretch that micro amount, with a hard glossy outer layer that would maintain the contour.

It would depend on exactly what the tolerances were and how temperature and humidity changes affected that.

I know that at the Reynolds numbers that a racer works the effects of turbulence are going to be much less than on a glider, but they are still big enough to go after. Whatever is done is going to be arcane and fascinating, or at least to this shop hound.
 

BBerson

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Port Townsend WA
I recall an article in SOARING magazine about filling a wood winged SF27M motorglider (30-40 years ago)
He spent about a winter filling. Then it didn't last, I think.
 

lr27

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Nov 3, 2007
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3,709
I've put 3/4 ounce glass and epoxy on a plywood boat that's been outside for years. It holds up very well except where it's been been mechanically damaged. I have some paint on it, but I've seen painted plywood with no glass start to "check" after a few years outside, even on a vertical surface that doesn't usually get rained on. 3/4 oz. glass is very thin, so if you plan on sanding into it, something heavier might be better.

I'd guess that epoxy that can handle higher temperatures than West might be a little more stable, especially under dark paint. But keep in mind that some epoxies adhere to wood better than others. I don't know what the best combination is.
 

Tantrum1

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Saint John, NB Canada
I would strongly suggest sanding the wood smooth and applying a layer of epoxy and light fiberglass, maybe 1 ounce or 2 ounce smooth fabric. Bake that at some temperature to fully cure the resin. When that is 100% cured, then put any filler and surfacing material on and sand smooth.

The reason for this is to plasticize the wood and prevent it from moving around as much a possible.... before you start madly chasing contours and smoothness. Otherwise you are shooting at a moving target.
That is exactly the plan. The original question was to just rid of a few valleys, and imperfections to prevent any sharp edges prior to the cover of fiberglass.
 

Victor Bravo

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Beware the dreaded "resin lines" if you use epoxy and microballoons as your filler. I have heard from one or two people that when you put epoxy/micro in specific areas ("valleys", low spots) the outer border of the filled area will collect a small line of pure epoxy resin around it. This is harder to sand than the main area... the epoxy can soak out or separate from the microballoons at the very edge. This makes it impossible to "feather out" or smoothly sand the edge of the filled area, because you have hard epoxy right next to "soft" micro. So you are always fighting the very edge of the filled area.

This might not be as much of a problem if you are eventually putting light glass over it... the problems my friend told me about were for final contouring and paint prep (Vari-Eze).

Also, my understanding is that the way to avoid this problem is to apply the epoxy/micro in one thick layer over the whole wing, so there is no "border" of the filled area. The problem occurs when you have a large smooth area, with a low spot in it,a nd you put a blob of filler into that spot. The "resin line" forms around that blob and makes it hard to perfectly blend it in to the surrounding smooth area.
 
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TFF

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Apr 28, 2010
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Memphis, TN
All the shiny straight auto restorations today pretty much fill the whole car. Some is for time. Feathering is time consuming to do right. You might have to do it 3-4 times to get it perfect. Shops tend to put lower payed people on that job, so they want it fool proof.

You would have to think long and hard on this, but it might be better to put a layer of 1.5oz glass on, then fill, then cover with 3/4oz glass. Capture all in the epoxy glass layer. It is counter on how most do it. It’s going to be a great wing no matter what. Probably one of the best ones made recently.
 

wsimpso1

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Saline Michigan
Beware the dreaded "resin lines" if you use epoxy and microballoons as your filler. I have heard from one or two people that when you put epoxy/micro in specific areas ("valleys", low spots) the outer border of the filled area will collect a small line of pure epoxy resin around it. This is harder to sand than the main area... the epoxy can soak out or separate from the microballoons at the very edge. This makes it impossible to "feather our" or smoothly sand the edge of the filled area, because you have hard epoxy right next to "soft" micro. So you are always fighting the very edge of the filled area.

This might not be as much of a problem if you are eventually putting light glass over it... the problems my friend told me about were for final contouring and paint prep (Vari-Eze).

Also, my understanding is that the way to avoid this problem is to apply the epoxy/micro in one thick layer over the whole wing, so there is no "border" of the filled area. The problem occurs when you have a large smooth area, with a low spot in it,a nd you put a blob of filler into that spot. The "resin line" forms around that blob and makes it hard to perfectly blend it in to the surrounding smooth area.
+ 1 on the above. Anytime you try to fill low spots and then fair them, you just build wavey surfaces because the edge of the low spot is rich in resin and cuts poorly, while the low spots have easily sanded low resin content micro, and they get lower when sanded. Bad juju.

I disagree on glassing over wood that has been profiled with micro. Glass and resin directly on wood will make a more stable wooden surface. Fill any low spots "below grade, scuff sand them, then spray a dark epoxy primer. Follow up as soon as it is dry to handle with enough really dry micro so that you can now profile in one step per the above. Sand with 36 grit and big sanding sticks for straightness and profile, then take the whole shape down until the dark primer begins to come up through the micro. Change to 80 grit and remove the big scratches. Then it is time for the five wipe coats of neat resin to fill all the pinholes. This scheme has worked great for countless boats and many airplane parts.

Billski
 
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