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Building Tips and technical articles. => Building techniques => Topic started by: Fred Quedenfeld jr on January 08, 2021, 04:12:13 PM
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Which epoxy do you use to apply balsa to foam?
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How much epoxy do you scrape off the balsa before you attach it to the foam?
Last time I did it I used Hobbypoxy one tube was yellow and the other blue
We all want light models and epoxy is heavy
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Drop Bob Hunt a message or email and I believe he has a file that he sends out that details this process/ You might even search this section of the forum as this has been covered many times.
Type at you later,
Dan McEntee
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I've heard that finishing resin is good for that. Mix per directions, apply liberally to the balsa, scrape off the excess (surface should almost look dry), and then apply to foam. Put foam cradle in place, weight it down, and let cure.
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Personally me never made balsa planking over whitefoam, but my comrades in the club made that professionally.
They used no epoxy, but a not-too-quick developing, self-expanding polyurethane glue. Sorrowfully I cannot advise you American brandnames, but you surely will find several brands. Choose medium one, not the quickest.
After painting it onto foam, slightly make wet the balsa with a sponge (some humidity is needed to start the polyurethane expanding (foaming).
After putting on the balsa sheets, close the whole wing between the original negative parts of whitefoam.
Push/press together by weights, or put the whole sandwich into a vacuum-sack.
The whitefoam in the halfwings must not be empty: even cut out the enlightments later, or put that parts back, before pressing.
This polyurethane glue will fill all the porosity of whitefoam, and it is perfect, very adhesive glue, however -being foam itself- the lightest choice.
Istvan
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I've heard that finishing resin is good for that. Mix per directions, apply liberally to the balsa, scrape off the excess (surface should almost look dry), and then apply to foam. Put foam cradle in place, weight it down, and let cure.
Careful with the nomenclature - *Epoxy* "Finishing resin" is what you mean. But "Finishing resin" used to be what they called polyester resin, which will eat foam faster than acetone.
Finishing/coating epoxy will work, but beware of it setting too quickly. If you mix it in a cup, it heats itself and greatly shortens the working time. I have even had it *boil*, then set, leaving shiny epoxy foam. If you use that, pour it out on some aluminum foil so it leave a big puddle, and stays cooler. It also tends to be very brittle after a while, on purpose, because it needs to set up rigid so you can sand it.
I use EZ-Lam "60-minute" from Aerospace Composites, it's reasonably thin, but it is also *much* tougher than coating epoxy and barely self-heats at all, and has extended working time.
Follow Bobbys directions - he uses hair spray to seal it first, others use nitrate dope, that keeps the epoxy from soaking in and reduces the weight. One important point - it takes *very little* glue, but a lot of weight, to make a good bond.
Brett
p.s. https://store.acpcomposites.com/custitem_acp_facet_type/Resins
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Thanks, Brett. I was trying to hurry to post, and left out some key info. You corrected me - thanks.
Do what they say (Brett and Bob) and you'll be just fine.
Steve
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Thanks, Brett. I was trying to hurry to post, and left out some key info. You corrected me - thanks.
I would call it more a clarification than a correction. I knew what you meant after I read it a few times, but you can't be too careful when giving people directions when they are just reading it.
Brett
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I always used 3M spray glue.
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I always used 3M spray glue.
I used to do that, too, but 3M77 now eats the foam due to some reformulation, was heavy, and prone to delaminating. Southern Sorghum was just heavy, otherwise OK.
I think any form of contact cement is pretty well obsolete , epoxy is much lighter and much more stable. And as noted above. polyurethane glue like Gorilla Glue seems promising , but I haven't tried it.
Brett
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Brett, Could be. I haven't built a foamy in a very long time.
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I used to do that, too, but 3M77 now eats the foam due to some reformulation, was heavy, and prone to delaminating. Southern Sorghum was just heavy, otherwise OK.
I think any form of contact cement is pretty well obsolete , epoxy is much lighter and much more stable. And as noted above. polyurethane glue like Gorilla Glue seems promising , but I haven't tried it.
Brett
When it came time to sheet all the foam on Pushy Galore, I decided to do a bit of research and testing first. The reason was the model was mostly all foam except for the fuselage.
Yes, the 3M77 has changed quite a bit and is no longer suitable to sheet foam wings.
I'm not sure if Epoxy is heavier or lighter than contact adhesives, I've never done any tests.
I never sheeted balsa to a foam wing using Epoxy and I had no interest in using Epoxy for various reasons.
I called around and spoke with a few competent contractors who do furniture restoration.
Two of them suggested the same aerosol can contact adhesive which is available at Home Depot. I will dig up the can if anyone is interested.
I was not going to use the product without testing it first. I had a piece of foam available, actually an old wing core from my R/C pattern days.
I followed the directions on the can, unusual for me, and sheeted tops and bottoms on two sections.
I actually mentioned this long ago when I inquired about sheeting the foam on this model.
Anyway, this was a few years ago and I'd have to look at the Pussy Galore build to see when I sheeted the model.
I put the sheeted samples outside in the Florida environment for 2 weeks. Exposed to the elements. Hot sun, rain and all you would expect.
2 weeks later I was shocked there was absolutely no separation of the sheeting or the foam.
The remainder of the can I used on this model.
Only the fuselage and the wing tip verticals are balsa, everything else is sheeted foam.
All this time, and the model has been in a hot storage space upstairs and in an air conditioned environment. Absolutely no separation and no changes.
The model is for sale BTW, in our Classifieds.
Choose wisely.
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I'm not sure if Epoxy is heavier or lighter than contact adhesives, I've never done any tests.
I *have*, as have many other people over the past 40 years. Contact adhesive is heavier and not suitable.
Brett
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I *have*, as have many other people over the past 40 years. Contact adhesive is heavier and not suitable.
Brett
Please post information on your test findings.
I'm sure many modelers would like to know the results.
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I have several sheeted foam wings from Tom Dixon. He indicated that he uses regular polyurethane Gorilla Glue for its ease of use, speed of cure and low cost.
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Please post information on your test findings.
Since you asked so nicely, no.
I'm sure many modelers would like to know the results.
They already know.
Brett
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I have several sheeted foam wings from Tom Dixon. He indicated that he uses regular polyurethane Gorilla Glue for its ease of use, speed of cure and low cost.
Without having tried it myself, it seems like a very good idea, and everyone who has tried it (and are qualified to assess the results) tells me it works very well.
If nothing else, it (and epoxy) solves the worst problem with contact cement - the fact that once it touches, it is stuck, and if it is warped when you do it, then, that is a shame. The idea is that you can stick it on, then put a lot of weight on it, leave it overnight, and it will "creep" despite the instant bond, to the right shape. That is, it will creep enough to conform to the cradles. I never had a problem with that, mine came out straight-ish enough, but I am very dubious that it will creep very much if at all. And I know that other people ended up with crooked wings that we couldn't ever diagnose.
Of course, if it will creep after contact the first day, at room temperature, what happens to it when you leave it in a 140 degree car all day? We know that answer, too - it delaminates.
Epoxy and anything other than contact cement will easily "slide" to conform to the cradles and should come out as straight as your table.
Tom Dixon and I have had our minor disagreements over the years, but he is certainly far more than qualified to tell whether or not balsa stuck to something, so it really boils down to how much it weighs.
Brett
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I use Kleiberit PUR 501 diluted with acetone to mixture of yogurt. After applying the glue, I wait 3-5 minutes, the acetone evaporates and does not corrode the foam.
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Without having tried it myself, it seems like a very good idea, and everyone who has tried it (and are qualified to assess the results) tells me it works very well.
If nothing else, it (and epoxy) solves the worst problem with contact cement - the fact that once it touches, it is stuck, and if it is warped when you do it, then, that is a shame. The idea is that you can stick it on, then put a lot of weight on it, leave it overnight, and it will "creep" despite the instant bond, to the right shape. That is, it will creep enough to conform to the cradles. I never had a problem with that, mine came out straight-ish enough, but I am very dubious that it will creep very much if at all. And I know that other people ended up with crooked wings that we couldn't ever diagnose.
Of course, if it will creep after contact the first day, at room temperature, what happens to it when you leave it in a 140 degree car all day? We know that answer, too - it delaminates.
Epoxy and anything other than contact cement will easily "slide" to conform to the cradles and should come out as straight as your table.
Tom Dixon and I have had our minor disagreements over the years, but he is certainly far more than qualified to tell whether or not balsa stuck to something, so it really boils down to how much it weighs.
Brett
Hi Brett:
I've tried the Polyurethane Gorilla glue and it works okay. You have to work quickly to insure that the stuff doesn't start to "foam up" before everything is indexed correctly in the cradle, and I've found that you can only do one side of a core at a time safely. That's way too slow for me. The weight is pretty much a wash between the Gorilla Glue and Z-Poxy when each are used correctly. I'd give the Z-Poxy a slight edge in that regard, though...
I've been covering foam wings with balsa commercially for more than 50 years (53 to be exact), and have tried dozens of adhesives. Heck, I used to have adhesive salesmen come to my shop to demonstrate their wares. I've tried all manner of spray adhesives, contact cements, polyurethane glues, and all sorts of epoxy - starting with good old Hobby Poxy. Nothing I've tried comes close to the success I've had with Z-Poxy Finishing Resin (part number PT-40). Properly used it is light, strong and has adequate pot life to do both top and bottom skins on a core at one time. I agree with you, Brett, that a flat table (or in my case a piece of 3/4-inch thick granite on a flat table...) and the epoxy method allows the skins to be positioned perfectly in their cradles and yield a perfectly straight wing.
Of course everyone will have their favorite methods...
Later -Bob Hunt
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Hi Bob;
My first exposure to skinning foam cores was back in my R/C sailplane days. All sorts of material was used and all sorts of glue, but for the real high stress, high performance wings, people were going to epoxy, and the favorite method of applying pressure was vacuum bagging. I won't bother explaining it because I'm sure you know what I'm talking about, but was curious if you tried it and what your experience was with that method.
Type at you later,
Dan McEntee
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Hi Dan:
Yeah, I'm very familiar with vacuum bagging, but I don't use it for skinning my wings. The weight works great and besides many of the wings I make have cored out front sections prior to covering to allow me to put landing gear ribs in place. They have to go in prior to the sheeting. If I were to pull enough vacuum to accomplish something that the weights don't, then I would most likely crush the front section of the wing. It has worked for me for all these years and I don't think I'll change my old ways at this point.
My son, Robby, has some ideas for which we will need the use vacuum bagging. A story for another time...
Bob
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Hi Dan:
(Clip)
They have to go in after prior to the sheeting.
(Clip)
Bob
? ??? :-\ n~ :!
Or, maybe I do not understand the obvious.
Keith
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Hi Brett:
I've tried the Polyurethane Gorilla glue and it works okay. You have to work quickly to insure that the stuff doesn't start to "foam up" before everything is indexed correctly in the cradle, and I've found that you can only do one side of a core at a time safely. That's way too slow for me. The weight is pretty much a wash between the Gorilla Glue and Z-Poxy when each are used correctly. I'd give the Z-Poxy a slight edge in that regard, though...
OK, that's interesting, I hadn't considered that the working time would be an issue. I have used epoxy ever since you told me to. BTW my minor contribution to the topic is that the EZ-LAM 60 minute seems to have extremely good pot life and working time without having to resort to the old HobbyPoxy 2 "make a big puddle on a cookie sheet" trick. Even in a cup, the temperature rise is negligible after an hour and you can easily still work it. It is also much tougher/less brittle than the finishing epoxies.
Brett
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? ??? :-\ n~ :!
Or, maybe I do not understand the obvious.
Keith
Yeah, yeah, you would think that after all these years I would remember to proofread and edit... n~
All fixed now - Bob
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I used to do that, too, but 3M77 now eats the foam due to some reformulation, was heavy, and prone to delaminating. Southern Sorghum was just heavy, otherwise OK.
I think any form of contact cement is pretty well obsolete , epoxy is much lighter and much more stable. And as noted above. polyurethane glue like Gorilla Glue seems promising , but I haven't tried it.
Brett 3M has the old formula available again. The new foam eating formula can be used but you need dust coats with the can held 18 inches away from the foam. That gives the chemicals a chance to evaporate a bit. The wood don't matter.