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Old Jan 22, 2008, 03:59 PM
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dusty IV's Avatar
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Epoxy resin vs polyu

I have a model planked in obechi. I usually use epoxy resin with fiberglass to strength this surface.

Some where in the distant past I think I read that you can use water based polyurethane. Does anybody have a comparison on this?
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Old Jan 22, 2008, 11:19 PM
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Troy's Avatar
United States, CA, Lake Forest
Joined Feb 1999
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Epoxy and glass will be stronger than the Polycrylic and glass, but the epoxy is harder to sand compared to the polycrylic. I have done it on several models and it is fairly easy to work with and easy to get a nice finish. The nice thing is that it is single component, easy clean up, and easy to sand/fill/primer. You will not get the durability of an epoxy/glass surface but I find it is easier overall to build lighter and gain some durability.
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Old Jan 23, 2008, 12:47 AM
DLD
David Layne
Tracy, Ca
Joined Jun 2004
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It kind of depends on what sort of internal structure your wing has. If it has a good solid spar system and doesn't need the additional strength, use poly, as it's lighter and easier to deal with, and will give a very nice surface. If you need additional strength, go for the epoxy finish, and maybe add some lightweight glass.

Good Luck,

DLD
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Old Jan 23, 2008, 12:41 PM
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Actually its on the nose of a sorta Kadet Senior. The 123 battery is very heavy and I wanted some extra strength. Planning on using 3/4 oz fiberglass over the Obechi. The Obechi is prone to splintering compared to balsa so it needs something to hold it together.

The plane gets covered in Oracover including the nose. The poly sounds like it might work so I'll give it a try. Maybe I should build up two samples before messing up the plane.
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Old Jan 25, 2008, 06:42 AM
Blue Collar DLG Marshall
Greensboro, NC
Joined Jun 2007
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In what you are describing the strength will come from the glass and the poly will simply keep it attached and solid. I use WBPU with a variety of materials to give my foam wings are hard skin. Not as hard as epoxy but, as mentioned, perfect in every other way.
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Old Jan 26, 2008, 03:38 AM
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Bakersfield, Ca.
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Hmmm.... Polyurethane and polycrylic are vastly different substances. Don't get them confused.
The urethane is a glue and expands as it cures.
The polycrylic is a type of varnish clear coat and not meant to be a glue though it does a respectable job of keeping the glass adhered to the substrate.
Marlan
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Old Jan 26, 2008, 06:09 PM
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But for finishing an exterior surface using single component Polyurethane (Gorilla glue eg) would be a mess. The water based polycrylic is a very easy system to use with light glass and easy to control weight and later prep for paint. I sheet wood over foam with Poly-U often with glass reinforcements but would never consider it on the exterior due to foaming and rubbery sanding surface.
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Old Jan 27, 2008, 01:19 AM
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Quote:
Originally Posted by Troy
But for finishing an exterior surface using single component Polyurethane (Gorilla glue eg) would be a mess. The water based polycrylic is a very easy system to use with light glass and easy to control weight and later prep for paint.
No issue there. It works well for that.
Marlan
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Old Jan 27, 2008, 01:56 AM
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Here is an example of how you Can use polyurethane(Elmer's Ultimate) and glass, if you have a mold.

http://www.rcgroups.com/forums/showp...5&postcount=33

I've built the pod using epoxy and glass as well. The PU and glass seems much less brittle after crashes and hard landings - still going strong with just scrapes and scratches. Your application is different, but just showing it can be used in molding.
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