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Finishing Epoxy, vs Regular Epoxy

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Old 03-26-2014, 05:26 AM
  #1
mikes68charger
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Default Finishing Epoxy, vs Regular Epoxy

Ok guys I have done a lot of fiberglass work in the past but it has all been with Plyester Resen, (Bondo Brand) with great results.

Anyways, Im working on a expoy wing, and Im a little confused. I have used standard HS Finishing epoxy for a repair with light glass. And from what I read from this site is that finishing epoxy offers no strength.

So If I was doing a repair, and I wanted to put one layer of standard fiberglass for strengh what epoxy do you use?

I tired 30min but it was to thick to be asorbed.

I plan on putting another layer of fine glass over this glass.

Thanks Mike
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Old 03-26-2014, 05:50 AM
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Ed Smith
 
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Polyester resin should not be used, it is very dangerous.

I have never used finishing epoxy for anything. I use regular epoxy for everything, molding parts or finishing a wing. I use MGS epoxy resin available from Aircraft Spruce and Specialty. It comes with two hardeners, fast (about 45 min cure) and slow ( about 6 hours) The hardeners can be mixed for a cure time of choice. Some experimenting is required.

I do not know quite what you mean by "Standard Glass". I assume you are glassing a balsa skinned wing. When I glass a wing I lay 3/4oz max glass cloth on the surface and squeegee the resin through it, ensuring there are no bubbles or wrinkles on the surface. This process adds very little strength to the wing. It does provide a hard surface for additional finishing. Do not lay on additional glass over an already cured surface. The result will be lumps, bumps and wrinkles, extra weight and no additional strength.

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Old 03-26-2014, 12:11 PM
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Mike depending on what you are doing will determine what resin type to use. I think finishing resin is more of a hobby definition as the term is never heard in the composites industry. To be honest, I keep a gallon of laminating epoxy resin around for all of my epoxy needs. I use it for laying up glass fuses, sheeting foam wings and when I need an epoxy glue I just add some milled fiber and cabosil to make a paste. I know you are trying to repair that jet wing, I just don't know how you are going about it. A little more info would help to be able to make solid recommendations.
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Old 03-26-2014, 03:46 PM
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With polyesters resins you have finishing and laminating variations. A finishing polyester resin has an added wax (or other chemical) that causes the resin to cure hard on the outside surface. Polyester resin has an air inhibited cure. The added wax migrates to the outside surface and seals it so it fully cures. A normal polyester laminating resin will cure with a tacky outside surface if it's exposed to the air. This is good because it will receive receive another coat or lamination without the need to prep the surface for bonding. It's a problem when it's the outside layer and you want to prep if for paint because you can't sand it worth a hoot. This is NOT the case with epoxy resins. Epoxy resins will cure nice and hard even if exposed to the air. Some of them will "blush" if the humidity is high. It can be sanded off rather easily. Pacer confuses the whole thing by calling their laminating epoxy a finishing resin. There is no such epoxy beast in the composites industry. In the composites world a finishing resin is a polyester with the added wax. Typically epoxies can be broken down into laminating and bonding variations. Laminating epoxies are for making fiber reinforced composites (fiberglass for example). Bonding epoxies are used for bonding parts together.

For your wing you want to use a laminating epoxy or a polyester finishing resin. I'm an epoxy guy, but a finishing polyester would do just fine in your scenario. The cure time is less with polyester, the cost is less, and the strength difference in this scenario is barely noticeable. Polyesters are stinky so be aware of your surroundings and be able ventilate the area. Epoxies can have a longer working time (pot life) which can be handy if your surface is large. You can always mix up a fresh batch if your present one starts to set-up in the cup. No need to panic.

Last edited by wyowindworks; 03-26-2014 at 03:54 PM.
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Old 03-26-2014, 06:45 PM
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I though polyester resin did not bond well to epoxy resin.

Im assume the wing is epoxy resin as its not green like my Byron f16
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Old 03-27-2014, 05:09 AM
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Mike, yes your wing is epoxy based however polyester need not be green. Usually it can be clear all the way up to a dark amber. For what you are doing I would make the suggestion to get some laminating epoxy. West System is about the most readily available but there are others that will work well too. Depending on how you are going about repairing the wing. I have done lots of repairs to molded wings that have de laminated by applying vacuum to draw resin in from an injection site to the vacuum site. This involves taping a bag directly to the surface applying vacuum and then injecting resin in just outside the bag. Not sure how well that would work with your situation, I still think it would be better to replace from the spar forward with a balsa sheeted foam core.
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Old 03-27-2014, 06:44 AM
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Just a quick note on Gougeon Bros epoxies such as West Systems and Pro Set (and I suspect others as well)....these are indeed readily available but I am beginning to see that the typical composite store sources are starting to charge extra handling fees for the hardeners and are charging even more for shipping as hazardous material (which is nonsense).

I did a quick search and found that Amazon has not only better pricing than the typical composite stores, but their sources don't charge the extra tarifs. S+H is often minimal, 5-7$ direct from Amazon's source. I'm sure nobody minds saving some greenbacks
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Old 03-29-2014, 06:58 AM
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Ok so let me make sure I got this. I can get http://www.jamestowndistributors.com...3A1+Epoxy+Kits

Then thin it down with alcohol for use as finishing epoxy?
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Old 03-29-2014, 02:19 PM
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You shouldn't need to thin, doing so with alcohol will actually reduce strength. If you need to get it to flow a little better just use a little heat.
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Old 03-30-2014, 07:02 PM
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I've been using Bob Smith Finish Cure finishing epoxy for many yrs and works fine for your purposes. No need to thin it as it's already thin enough for glassing. The strength is in the composite between the glass and epoxy. The "other epoxy" is just epoxy glue and not meant to be used with glass.

Last edited by Flypaper 2; 03-30-2014 at 07:06 PM.
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