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do i really need to use glass

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Old 05-30-2003, 10:26 PM
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highflyer353535
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Default do i really need to use glass

i have decided to fully sheet a cg extra and paint it instead of covering. do i really need to glass it before shooting it with the paint? or can i use some other method to stiffen it before painting? any help will be greatly appreciated thank you
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Old 05-30-2003, 10:49 PM
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David Cutler
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Default do i really need to use glass

Adding glass will considerably increase its strength.

The mechanism that makes fiberglass so strong goes something like this:-

In a properly laid up fiberglass structure, the strength of the interface between the glass fibers and the matrix (in our case, the epoxy glue) is designed to be less than the strength of the fibers themselves.

All surfaces have micro cracks in them, which provide a very localized high stress point for any break to propagate through. (a bit like using your teeth to start a cut in a plastic wrapper before tearing it with your fingers) Now, if that growing mini-crack meets a glass fiber, it stops traveling in the original direction of the crack, and the strain breaks the joint between the matrix and the fiber, (as it's weaker than the fiber itself) thus changing its direction along the length of the fiber. Since the crack is now propagating in an entirely different direction from the strain that created it, it tends to dissipate and disappear.

This keeps happening until the strain gets spread across a larger surface area than the micro crack.

That's why, when a fiberglass structure finally fails it sort-of explodes as the stresses are greater than they would be in a homogeneous structure.

Argh! Writing that reminded me of my mech engineering course back in the dark ages!

-David C.
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Old 05-31-2003, 01:13 AM
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Flypaper 2
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Default do i really need to use glass

Micafilm is another option. leaves a smooth finish and takes paint well. My avatar is covered with Micafilm.
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Old 05-31-2003, 07:52 AM
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Steve Lewin
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Default do i really need to use glass

Tissue and dope is the old-fashioned equivalent. But whatever you go for you have to put something over the wood to get rid of the grain and hold it all together. Just painting doesn't work at all well.

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Old 05-31-2003, 01:01 PM
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Al Stein
 
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Default do i really need to use glass

I've built balsa glider fuselages on which I filled the grain with thinned lightweight spackle, primed with automotive filler-primer and painted (obviously, I hope, there were sanding, surface checks, and repriming steps in between).

The finish comes out smooth as glass, but the structure underneath better be strong enough for whatever the plane's going to be subjected to (many of our planes take part of their strength from the covering), and the finish is easily dinged or nicked. Fiberglass is good stuff and amazingly light when done well, and it gives you a result that goes beyond looking good like my unglassed planes do... it gives a surface that keeps on looking good. It's a good thing.
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