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Old 03-05-2012, 11:15 AM   #11 (permalink)
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Originally Posted by skyking View Post
The glad wrap easily wrinkles and does not transmit a flat surface onto your job. It does work as mold release and I use it to put under parts I don't want stuck to the table.
I have had great luck with it.

I lay up the glass, impregnate with resin, stretch the wrap over the epoxy, and tape it down beyond the wetted area to keep it tight.

And it's stretchable, so it conforms to 3D surfaces.

Works good.

If it wrinkles, then something is shifting and causing the wrinkle.

Jim.

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Old 03-05-2012, 11:23 AM   #12 (permalink)
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how do you work the air out from under it? I use a plastic squeegee with the mylar and heavier films.
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Old 03-05-2012, 11:29 AM   #13 (permalink)
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you can also spray with gellcoat, then PVR over the top. the PVR seals the air off so the Gellcoat will kick.

all of these methods leave a thin coat, which is great if you are dimensions are very close to final. My problem is when I am building something new, I often have to take 1/8 to 1/2 inch off to make the body right, which means I need to eat into the glass.

And for that, I sand down past where I want, then skim it with bondo, lots of elbow grease with sandpaper to make it smooth, then paint over it.

Most of this to make the plug of course as once you have a final product, you can make a mold and from then on the bondo is needed a lot less.
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Old 03-05-2012, 12:30 PM   #14 (permalink)
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Originally Posted by skyking View Post
The glad wrap easily wrinkles and does not transmit a flat surface onto your job. It does work as mold release and I use it to put under parts I don't want stuck to the table.
This wrinkling (smooth waves really) is what I found as well with the 4 mil poly . But I did not tape it down beyond the wetted area like 3 wheeler suggested. Sooo I will try again.

Hope this thread continues as these tricks and tips are hard to dig up on the web. Especially the fiberglass / foam info. By the way I think the best book I found on Fiberglass/foam is "Advanced Composite Techniques" by Zeke Smith
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Old 03-05-2012, 02:39 PM   #15 (permalink)
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By the way I think the best book I found on Fiberglass/foam is "Advanced Composite Techniques" by Zeke Smith
Rutan's Moldless Composite Construction is another pretty good one. The Gudgeon brothers have put out a magazine for years with interesting tips, too... can't think of the name, but the WEST system epoxy site would have links.

Another issue with poly film it that is is rarely smooth in the way that mylar is. Even stretched tight, its surface has little bumps and waves (near microscopic) that show up on a glossy surface.

For those interested in very lightweight with foam-cored composites, standard vacuum bagging has the disadvantage of leaving a lot of resin in the foam surface pores. If the thing you are making permits using panels as you would use thin plywood, then you can squeegee fiberglass cloth in epoxy onto a large piece of plate glass (treated with a good mold release). Then, prime one foam side with microbubbles in epoxy, which can be about half the weight of epoxy alone. Flip this onto the table over the fiberglass. Vacuum bag and cure. Peel the panel off the table. Make the second skin the same way.

The resulting panel is perfectly flat and shiny on both sides, and can be single curvature bent into place (the curvature depending upon skin thickness vs core thickness, core stiffness, etc etc.) onto a structure, such as an airplane wing (albeit not around the leading edge) maybe about from 15% on back.

A little extreme in terms of labor to save some epoxy weight.
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Old 03-05-2012, 03:07 PM   #16 (permalink)
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Originally Posted by drmiller100 View Post
My problem is when I am building something new, I often have to take 1/8 to 1/2 inch off to make the body right, which means I need to eat into the glass.
The people who have built Vari-Eze planes have gotten pretty good at this. The idea is to get the foam nearly perfect in shape, before using any filler at all. This is right about at my patience limit -- especially on something like a wing that has to be a particular size at each point. On stuff like an autobody, if you sand too deep, just go to the other side of the car and make it too deep there too. If you need to, make up a name for that aero feature -- any German name should do. I used to have a windsurfer with an Ophibi dent or some such... to promote earlier planing. I think they just dropped the plug and didn't want to fix it.

(You can translate the stuff below for polyester -- it can be a little different, and I rarely use it, other than as Bondo) Once the shape is right, coat with microbubbles in epoxy, and lightly sand that. If there are places where you've gone through the micro, recoat and sand some more. Then lay on the cloth and squeegee on the epoxy. Unfortunately, this is when the waves show up that you didn't see when it was micro surfaced, which is flat, not glossy. I've painted things with glossy paint prior to glassing just to see the surface better. Then that can be sanded to matt to allow the epoxy to bond well. But the better the surface prior to any glass, the easier things go.

But I have never really gotten the hang of it, anyway. However, on boats, where the substrate is wood, this works more easily. One sanding after the glass goes on, followed by rolling on another coat of epoxy, followed by one more sanding is usually enough that the boat can be varnished without any filler used at all. (If you need to use filler, you simply convince yourself that you really wanted a painted boat anyway.)

BTW June sounds good. I'll reply also wherever you mentioned the trike fest.

Last edited by Ken Fry; 03-05-2012 at 03:21 PM..
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Old 03-05-2012, 03:27 PM   #17 (permalink)
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But I have never really gotten the hang of it, anyway.
Oh, another thing that can be helpful: When you have large areas to fill and fair with a surrounding surfaces (I've done this on sailboat keels, which are airfoil shapes that people obsess over) is to apply the first layer of filler with a notched trowel. (I think West system sells small plastic ones for this purpose.)

Then, sanding to shape is pretty easy, because you don't have to sand so much stuff. The second layer of filler fills in the valleys and the sanded ridges guide the squeegee, so very little final sanding is required.
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Old 03-06-2012, 01:04 AM   #18 (permalink)
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all good points, Ken. I like the notched trowel trick.
I tried the the poly film tonight, it was a no-go for me. Not enough hands and too big a part. They don't call it cling wrap for nothing
somebody asked about a supply for mylar. I stopped at the wholesaler, and the best they could do was $200 for a 200' x 53" x .005!
I had hoped for better. She is checking on some 15 mil HDPE. IT *might* be stiff enough to work.
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Old 03-06-2012, 03:14 AM   #19 (permalink)
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mostly what I have figured out is I can dream BIG, and throw something BIG together. Then I need to hire someone to make the waves, holes, bumps, etc all go away.
Those I hire are scared to think BIG - they can't dream of making the overall shape, but once I have a REALLY UGLY shape laid out, I can find people to sand and bondo for hours and hours for only 5 bucks an hour.
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Old 03-06-2012, 09:18 AM   #20 (permalink)
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I can find people to sand and bondo for hours and hours for only 5 bucks an hour.
That can be a really good solution. DIYers can be their own worst enemies, at times, by failing to turn over parts of the project to others.

I generally don't weld stuff together that goes to a customer -- I just don't seem to have the level of skill anymore. Good enough for my own stuff. I find it a little hard to to turn welding over to an artiste, but admire the results.

The POC has a mix -- all the steel done by me, all the aluminum done by an artiste. Occasionally, someone will say "Wow, did you do all the welding?" and I have to fess up: "Yep, all the ones that look like wads of bubble gum. I hired out all the good-looking ones."

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