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  1. #1

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    Mixing Epoxy and Polyester in a cowl?

    I made my first cowl using West Systems 105, and while it came out pretty good for my first one, it seems a bit too flexible. I was wondering if I used Epoxy as the gel coat, and for the glass layer use Poly resin to stiffen it? My friend made a cowl for his Cessna out of poly and it is much stiffer than the Epoxy is, but seems like it would be more likely to crack. I would like it a bit stiffer than it is, but more flexible than the plain poly. I used .5 oz cloth followed by 1 oz for the layers, and it weighs in exactly the same as the plastic cowl I modeled it from, though once the vents and the hole for the prop are removed, it will be lighter since that is where most of the weight is in the epoxy.

    So can Poly be layered over Epoxy and bond, or will it separate after a while? I have 2 quarts of poly that needs to be used up, so looking for projects for it. Have 8 B-17 cowls to make next, and getting some practice in on the Chipmunk mold I made.

    On the flip side, could it be done this way to make the mold?
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  2. #2
    speedracerntrixie's Avatar
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    No No No..........the only time you would EVER want to mix up these two systems is when you are using epoxy to bond parts into a correctly prepped polyester based layup. West systems is mix and cure temp sensitive. Usually a scale is a must to get accurate mix ratios. I don't know how you mixed it so that is nothing more then an information tid bit. The next is temp while curing and I personally never allow a part to cure at anything less the 70 degrees. If I do an ambient cure on a molded part I always follow up with an elevated post cure of 180 degrees for 2 hours. For your part at this point I would recommend a post cure of 150 to 180 degrees for 3 hours. When finished it will have the stiffness you want.

  3. #3

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    In addition to the previous post. Most Polyester resins will not cure over Epoxy.

    Ed S

  4. #4
    sensei's Avatar
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    Quote Originally Posted by Ed Smith View Post
    In addition to the previous post. Most Polyester resins will not cure over Epoxy.

    Ed S
    Not that it matters, but I have never seen a properly mixed polyester/catalyst resin system that would not cure over top of a completely cured epoxy matrix or any other matrix for that matter. just saying...

    Bob
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  5. #5

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    I used the West Systems metering pumps. So using Poly over Epoxy=no. Got it. But can I use poly in the mold made from epoxy? Or Epoxy in a mold made from Poly?
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  6. #6
    sensei's Avatar
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    For room temperature cures; absolutely to both questions.

    Bob
    Last edited by sensei; 10-27-2013 at 11:41 AM.
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  7. #7

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    Thanks, I know what I will use the stinky stuff on then.
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  8. #8
    speedracerntrixie's Avatar
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    Did you try an elevated temp post cure on the part you have?

  9. #9

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    It has been a few weeks, think it will still work?
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  10. #10
    sensei's Avatar
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    Yes.

    Bob
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  11. #11

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    Quote Originally Posted by sensei View Post
    Not that it matters, but I have never seen a properly mixed polyester/catalyst resin system that would not cure over top of a completely cured epoxy matrix or any other matrix for that matter. just saying...

    Bob
    Bob,

    No doubt true. However I was not prepared to experiment further.

    I can speak only of my own experience. I once joined several balsa skinned foam wings with epoxy. I then glassed the wing using polyester. The polyester did not cure right along the joint line. That was the last time I used polyester for anything.Click image for larger version. 

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    Ed S

  12. #12

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    Quote Originally Posted by Ed Smith View Post
    Bob,

    No doubt true. However I was not prepared to experiment further.

    I can speak only of my own experience. I once joined several balsa skinned foam wings with epoxy. I then glassed the wing using polyester. The polyester did not cure right along the joint line. That was the last time I used polyester for anything.Click image for larger version. 

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    Ed S
    I have had that happen over CA joints too. Turned out to be the hardener had expired. For some reason the balsa absorbed enough rein for the hardener to kick in, but wherever there was a CA joint, it stayed tacky. I bought a new tube of hardener and ran a swab soaked in it across each joint and a few days later the resin hardened enough to be sanded smooth.

    I also had a quart of resin and tube of hardener that was stored for years in the basement, and tested it out. The resin got thick, the hardener didnt work at all, but a fresh tube let me use the resin for projects, and being that it was thicker it stayed in place on a vertical surface without adding anything, so a plus for that. Been using poly for car repairs for a long time, just started getting into making RC parts with it, and this is my first use of WS Epoxy and really like it.

    BTW Hair spray does work as a mold release, but is not as easy to use unless you get a pump bottle and pour it into an airbrush, as it goes on in larger splotches, but if ever in a pinch and run out of PVA, a quick trip to the nearest store can get you back in business. My PVA finally arrived so I can start my second Chippy cowl and see if I can get a smoother finish than the first one. I rushed the gel coat by not letting it set up first, and put it on too thick in places so I have runs. Also want to try the heat curing too. Should that be in or out of the mold?
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  13. #13
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    I stopped using polyester resins nearly 20 years ago, there are just to many great epoxy systems out there.

    Bob
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  14. #14

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    Quote Originally Posted by sensei View Post
    I stopped using polyester resins nearly 20 years ago, there are just to many great epoxy systems out there.

    Bob

    When you are one of those who prefers to go to a store and buy whats on the shelf, hard to find items like West Systems, become unknown until you hear about like I did on the forums. I am switching to Epoxy now and convinced my friend to do the same thing. He makes PT boat hulls that are about 5' long. They are to scale, and need to be hard, so his hold back is the flex he saw in my cowl, but he uses much heavier glass, and his cost for what he has been using was over 2K, and what the WS cost appears to be lower per finished product, but if the parts need to be heated to cure hard, that could be a hold back on using it. Though does give me an idea on making an autoclave.
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  15. #15
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    No need to go autoclave for hobby stuff. The elevated temp cures can actually be done easily. In my case I like to use free heat sources. Having a cowl set for a couple days in the car works rather nicely. For the months that are a bit on the cold side nothing beats a plywood box with a light in it. It doesn't take much heat anything between 100 and 175 will work. You boat friend may want to check into using some core materials. Using core materials during construction will lower the amount of cloth and resin used, increase stiffness and lower weight. Being a boat less weight is not really needed but if he ships these boats it may save him even more money. Using core materials will mean that he will have to vacuum bag during cure but the end result of that is a better quality finish. When shopping for resins you may want to check out Aircraft Spruce and Specialty. The get the stuff out quickly, have great tech support and good prices.

  16. #16

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    Quote Originally Posted by speedracerntrixie View Post
    No need to go autoclave for hobby stuff. The elevated temp cures can actually be done easily. In my case I like to use free heat sources. Having a cowl set for a couple days in the car works rather nicely. For the months that are a bit on the cold side nothing beats a plywood box with a light in it. It doesn't take much heat anything between 100 and 175 will work. You boat friend may want to check into using some core materials. Using core materials during construction will lower the amount of cloth and resin used, increase stiffness and lower weight. Being a boat less weight is not really needed but if he ships these boats it may save him even more money. Using core materials will mean that he will have to vacuum bag during cure but the end result of that is a better quality finish. When shopping for resins you may want to check out Aircraft Spruce and Specialty. The get the stuff out quickly, have great tech support and good prices.


    Ill pass it along. He usually does this stuff during winter when we dont have decent flying weather and its too cold to work on the Rat. He does have a large vacum forming table to heating plastic and forming parts. He has a couple hulls made, but is working on the deck furniture now and cabins. He can use up the ploy he has left for the cabin molds and we can experiment with Epoxy on the smaller parts. We may eve build us a vacumm table for bagging cowls since we have a few plans on the shelf needing parts.
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  17. #17
    sensei's Avatar
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    Quote Originally Posted by acdii View Post
    When you are one of those who prefers to go to a store and buy whats on the shelf, hard to find items like West Systems, become unknown until you hear about like I did on the forums. I am switching to Epoxy now and convinced my friend to do the same thing. He makes PT boat hulls that are about 5' long. They are to scale, and need to be hard, so his hold back is the flex he saw in my cowl, but he uses much heavier glass, and his cost for what he has been using was over 2K, and what the WS cost appears to be lower per finished product, but if the parts need to be heated to cure hard, that could be a hold back on using it. Though does give me an idea on making an autoclave.
    You will never regret making the switch.

    Bob
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  18. #18

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    Quote Originally Posted by sensei View Post
    You will never regret making the switch.

    Bob

    You can say that again just based on odor alone. You really have to breath deep to smell the Epoxy but you can smell that poly at the other end of the house. Thats why I want to use it up, so the hardener doesn't go to waste, and at least get some use out of it. I have just enough glass and mat to make one more mold and will probably use the poly for that, just to get rid of it. I can experiment with a few additives and see how they cook. The plug will be aluminum, polished to a shine, and waxed quite well using real PVA. Since its a round cowl, and not very large I can make it a one piece instead of split. After that though its epoxy all the way, once I compared the costs, it is far less expensive since less is needed. I was very surprised at the flexibility though, and not sure if I want to to be like that or stiffer. I think I will wait until I get the engine mounted and see how it clears, if I need to flex it to attach it I will leave it, if not I will harden it in the oven a couple hours as suggested.
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