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Trying to build a fiberglass cowl - need advise

Old 04-03-2004, 11:53 AM
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Default Trying to build a fiberglass cowl - need advise

Hi all, I am in the process of building a cowl for a plane I am building. I started out thinking of building a balsa cowl but now I am considering using fiberglass.

I am thinking of making a plug out of styrofoam so I can shape it to make it look the way I want then applying the glass over the foam. I'm not sure what exactly I should use for foam. I was thinking of using the blue home insulation type available at home improvement stores. Will this work or will I need a specific type of foam. Also, what is the best way to cut it. I have heard about the hot wire method but don't know anything about it. Otherwise I figured a kitchen type knife for rough shaping with sanding to get the final shape.

My last question is what type of resin is easiest to use for applying the fiberglass. Many say epoxy is too thick so I am trying to avoid having to mix different materials. Anyone tried polyurethane finishing material like that used for sealing wood? It sands nicely and dries clear so I thought it might work OK.

I don't mind spending a little time shaping the plug to get what I want. I just want to keep it inexpensive and simple and stay away from specialized resins, etc. I would prefer to use materials I can pick up at a hardware store if possible.
Old 04-03-2004, 03:58 PM
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Default RE: Trying to build a fiberglass cowl - need advise

Hi there

i have just made one leg for my profile taco, the first time i ever worked with fg.
I tried it a couple week back with epoxy but that doesnt work good at all. Today i bought Z-poxy finishing resin and boy what a difference. I really advise to use a sort of finishing resin or laminating epoxy. Its very thin and really 'sinks' into the fg. I used blue insulation foam as a mold, that worked fine. I did cover the foam with some plastic so I dont need to make two molds. I have heard of some resins melting foam but I believe that was with pink foam, dont think there are any problems at all with blue foam. I used a scroll saw to shape my mold but I suppose that a cowl has a much more complex shape so you will need to do it a little different. I would roughly shape it with a scroll saw and then use sandpaper to get the shape you like. I dont think a knife will work that good, its a bit risky when it slips and ruins the 'mold in progress'. Blue foam is very cheap so you can experiment a bit with different tools to shape it. I dont know anything about polyurethane so I cant help you there.

I will send a pic of my mould later

Hope this helps
Old 04-03-2004, 06:13 PM
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Default RE: Trying to build a fiberglass cowl - need advise

Thanks barto, I don't have a scroll saw yet but I do have a small coping saw that may do the trick. Like you say, I will have to test it to see how it works and the blue foam is cheap. I am hoping to keep the foam from getting too eaten up from the cutting process so getting a smooth surface won't be too difficult. That is part of the reason why I was considering the hot wire cutter.

I would be interested in the pics and how your final parts turn out.
Old 04-03-2004, 08:33 PM
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Default RE: Trying to build a fiberglass cowl - need advise

FlyerBry- I use blue foam for making plugs, it works very well. Although I do have a bandsaw I could use, I cut it with a razor blade. Get some 1/2 blue foam, and draw the shape of the cowl onto a lot of pieces, thick enough to make your entire cowl and then about 1 inch extra. Glue them all together with 5 minute epoxy, you don't have to cover the whole piece but focus on the very outside and the middle. Once they're all together, sand the foam to shape and fiberglass it using epoxy resin and a few layers of 1-2 ounce fiberglass. Sand the crap out this till it's smooth and shiny, then wax up, make your mold halves and make your part. We'll help you more when you get to that step. Epoxy is actually what most people use, whoever said epoxy is too thick may not know exactly what they are talking about . You need to get an epoxy finishing or laminating resin, though. Like Barto said, Zpoxy finishing resin is OK if you're just starting and you don't need a large amount or absolute strength. A cowl isn't structural so something like West Systems or MGS wouldn't make a big difference. BTW, Zpoxy is epoxy too.

Dan
Old 04-03-2004, 10:30 PM
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Default RE: Trying to build a fiberglass cowl - need advise

Thanks skypro, is Zpoxy something that is typically available at a hobby shop? It seems to be mentioed more than other brands. I was thinking of stopping at a local boat shop to see if they sell some type of finishing resin.

OK, you have me curious. You said make the mold halves... I'm guessing you are referring to making a female shell that the final cowl is created inside so it ends up with a smooth outer surface. Am I right? What do you make the mold halves out of? Is this out of fiberglass as well?
Old 04-03-2004, 10:40 PM
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Default RE: Trying to build a fiberglass cowl - need advise

I started using polyurethane foam a couple of years ago, and have kind of a love-hate relationship with the stuff. This is the stuff that is used as a base for flower araingements. It is brittle and you can carve it with your fingers, but drywall sand paper works well. Sculptors love the stuff. You can also use polyester resin on it.

Now the bad news. It aint cheep (or I just haven't found a good source yet). And it's MESSY. It leaves a fine, sticky (static cling) dust. I use a couple of 3 mil plastic tarps to curtain off the area when I work it, and it still gets everywhere.

For smooth curves like turtledecks I use wire cut white foam. Simple compound curves, like canopys, I use blue foam. I find it easier to sand than the white stuff. For more complex shapes like cowlings I use the Polyurethane. It is really easy to carve scoops, air inlets, etc.

As for resin, if you break down the price per ounce, you will find that West System is not that much more than Z-Poxy. It's just that West System, MGS, etc are packaged bigger.

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