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Amount of epoxy to mix for skinning wings

Old 11-09-2006, 12:49 PM
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sportflyer-RCU
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Default Amount of epoxy to mix for skinning wings

I have found that 1 1/2 oz of mixed epoxy ( EZ LAM) is more than enough for 450 sq inches of balsa wing skins provided the balsa skins are sealed with shellac or similar sealent and scrapped off well with a plastic card. I learned this the hard way when I overestimated the amount to use and so wasted about 2 oz of mixed epoxy. Any other guidelines ?
Old 11-09-2006, 09:31 PM
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Newc
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Default RE: Amount of epoxy to mix for skinning wings

balsa wing skins provided the balsa skins are sealed with shellac or similar sealent
That's completely eliminating the reason for any bonding agent, especially epoxy, which is required to be absorbed by the balsa to be most effective.
Old 11-09-2006, 09:47 PM
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sportflyer-RCU
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Default RE: Amount of epoxy to mix for skinning wings

Lots of people apply a sealant to the balsa skin before applying epoxy. Personally I have never had any delamination of the balsa skin using this method. Some modelers mist a layer of hair spray as a sealant before applying the epoxy, others use shellac as I do.
Old 11-09-2006, 09:48 PM
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Default RE: Amount of epoxy to mix for skinning wings

If you want to build light models (they do fly better, after all), you look for ways to maximize the use of your materials, and minimize the amount of excess weight. Sealing balsa with a light coat of sanding sealer for this application is very common in competition circles. The surface is still plenty rough for the epoxy to make an excellent bond... but the porosity is limited so that the resin can't "wick" into the balsa to the same extent. This means more of the applied resin will remain at the bondline... which in turn means less epoxy can be used. And that means less weight for equivalent strength. Certainly epoxy-saturated balsa is stronger than raw balsa... but the increase in strength is more than ofset by the disproportionate increase in weight. You are better off using a layer of fiberglass if you really need the extra "strength."

HTH,
-David
Old 11-10-2006, 07:09 AM
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Newc
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Default RE: Amount of epoxy to mix for skinning wings

The difference in techniques may be due to other differences in sheeting. I use the vacuum bagging method, and when doing this the epoxy is first spread out over the entire balsa surface and then as much as can be removed is taken off by scraping. Therefore very little - but enough for a strong bond as a result of the vacuum bagging - is left to add weight.
Old 11-10-2006, 02:10 PM
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sportflyer-RCU
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Default RE: Amount of epoxy to mix for skinning wings

I think both methods are acceptable depending on preference. The amount of epoxy to use is actually very little . I vacuum bagged the the second wing half today and found that 1 oz of mixed epoxy is more than enough to cover 450 sq inches of sealed balsa.
Old 11-10-2006, 04:23 PM
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Default RE: Amount of epoxy to mix for skinning wings

ORIGINAL: Newc

balsa wing skins provided the balsa skins are sealed with shellac or similar sealent
That's completely eliminating the reason for any bonding agent, especially epoxy, which is required to be absorbed by the balsa to be most effective.
Well, actually, no. The balsa doesn't need to 'absorb' any epoxy at all to be effective.

It is a common misconception that resin bonding systems must be 'absorbed' to develop their adhesive strength. As a matter of fact, the WEST System epoxy people, the Gougeon brothers, originally claimed they got the 'WEST' name from "Wood Epoxy Saturation Technique". They used that terminology extensively in their advertising. Go ask them about that now. They ABSOLUTLY will refuse to claim any such 'saturation' at all and they will vehemently try and distance themselves from their past claims... because as they became more knowledgeable about their own product, it became clear that no saturation, and hardly any absorption was taking place at all. Further, you can literally successfully bond two pieces of steel together, and there is no question of absorption in that application... but they are bonded nonetheless.

Tom

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