James c harrell

My Feedback: (18)

Let me start by saying this is my first attempt at this so bear with me. I am building a plane that has a foam wing with the leading edge and the trailing edge covered with balsa and ribs that are glued along the span to join them. I wasn't impressed with the strength of it at all when I got dome with it and decided to put in a spar and to put some fiberglass cloth with epoxy finishing resin. I wacthed many videos, which by the way made it look simple, but it ain't quite so easy. First thing was the resin I used, Great Planes finishing resin, was alot thicker than that I had seen. Very sticky and hard to work out wrinkles etc. I put down a light coat of it first, squeeged it as thin as possible, layed down the cloth, .56 oz, and put down another coat on top. I finally got it to where it looked half decent and am waiting on it to dry. When I do the other side of the wing would it be easier to heat the epoxy up a little to make it thinner and easier to work out? Any other tips that will help me would be a great help!

VIRGINIA MODEL BUILDERS

My Feedback: (159)

Rmember one thing before you heat the resin. Heat will accelerate the set up time and you will have to work faster than usual.

James c harrell

My Feedback: (18)

A good builder just told me that all I need to do is to set the resin in a bowl of hot water and keep it there. It will get thinner but not too hot as to affect the set time as bad. Sounds like good logic to me. Just wish he had told me that befor I started. He said now I would remember!! Only one way to learn this stuff. Do it!

wyowindworks

A 10*C (18*F) temperature increase of the resin will cut the working time in half. If you do more of this type of work in the future you might consider trying a true laminating epoxy resin.

James c harrell

My Feedback: (18)

which type do you recommend?

wyowindworks

Resin Research Composite Pro is a nice medium grade resin with good wet out properties. It comes in somewhere around $75 per gallon. West Systems is available at quite a few marine shops for around $100 per gallon. Adtech 820 is a nice resin with good wet out properties and has a higher HDT than Resin Research and West System. It's around $100 per gallon. I use mostly MGS L285 which is a premium epoxy with fantastic properties and is around $200 a gallon. Proset low viscosity resin\ (117) is another good choice and around $200 per gallon. West Systems is my least favorite of all of these but is often used by builders because its readily available from local marine shops. BTW, the prices include the hardener.

James c harrell

My Feedback: (18)

Are they Foam freindly?

wyowindworks

Yes. Most epoxies are foam safe. It's polyester and vinyl ester that are the problem.

kenh3497

In the past I have had great luck thinning the epoxy with denatured alcohol. You can make it as thin as you like. Some will argue it makes the epoxy "weak" or at least not as strong as it used to be..... more on that later.....

When I use the thinned epoxy I thin it so it wets out the cloth very easily. I have found the amount of the denatured alcohol added is not critical though I suppose you could add to much. I've gone as much as 50% without problems. I use a playing card to spread it as this is flexible and disposable. How do you know when you have the correct amount of the thinned epoxy on? Easy, if you have to much on the surface will look shinny and wet. If the cloth is "dry" it will not show the wood like the "wet" does. I know this sounds goofy but it will look dry If you have a wet spot just squeegee the wet until you move the excess to a dry spot or off the surface entirely if needed. After it cures, lightly sand with some 120 git being careful to not sand through. Apply another coat in the same manner and if you wish add some micro balloons to help fill the weave of the cloth. Sand smooth and use your favorite painting methods from here. Back in the day I built an all wood 1/4 midget racer. Glass cloth and epoxy on everything. I came out 4 oz under the minimum weight limit ready to fly. I had to use a 500 mil battery when every body else was using a 250!!

Now about the weakening of the epoxy. IMO if you are using the epoxy in a structal aplication I would not thin it in any way shape or form. The idea is to stick the glass cloth to the desired surface. The denatured alcohol evaporates pretty fast from the mix as you are applying a VERY thin coat. The strength of this system comes from the glass, not the epoxy. A side benefit is the evaporating alcohol cools the epoxy giving you a bit more time to get it "perfect". You will find the epoxy regains it's former viscosity pretty quickly so while "cool" it may get "thick" on you. try it on a scrap piece first to get an idea of how it goes.

About that 1/4 midget... I just sold it to a guy that wants to convert it to electric. It looked as good as the day I finished it except for 20+ years of hanger rash.

Good luck!!!

Ken

James c harrell

My Feedback: (18)

I have finnally got this thing sanded and after my arm recovers I will put another coat of epoxy on it. I think I kinda got a little to close in a spot or too!I was told at our club meeting last night by the best builder there the same exact thing you just said about the glass and strength. Thin it will be this time. Putting this stuff up there as thick as it was is a whole like work sanding it. Thanks for the advice!

wyowindworks

Thinning can work for very thin (thickness) applications that aren't significantly structural.

Here are the results of a test that was done showing that adding 5% denatured alcohol by volume can decrease the strength by nearly 50%. Also, the shrinkage of the resin goes up which will increase the internal stress of the laminate which promotes structure warping.

Thinning Epoxy with Solvent

kenh3497

Very good info wyo. My take is they are more concerned about thick pours. In any case over thinning could be bad. My experience with denatured alcohol has been good with no ill effects to date. Maybe in the future, who knows??? I will continue to use alcohol when glassing surfaces. You mileage may vary.

Ken

James c harrell

My Feedback: (18)

I think the finish epoxy I got must have been old or something. It was way to thick. It didn't cure well either. Took over 24 hours befor I could sand decent without getting little rolls. Any way I got it sanded and was afraid that I may have gone through in a spot or two. There is foam showing in the middle between the leading and trailing edges between the rib strips so I thinned the last of the epoxy with alcohol till it looked like the stuff I have seen on the videos' I have seen and brushed it over it and used a card to scrape it down. Looks a whole lot better now. Like glass! Hope it dries like that. Heck, I just hope it dries!!I guess we'll see tommorrow. I did find some stuff I like in this ordeal. I used some Pactra primer on the fuse and I like the way that stuff works. I had some that was in a box of stuff I picked up at a sale or somewhere, not really sure. But I got four cans of the stuff so I tried it. Sands down nice and smooth.

wyowindworks

As long the application is thin enough the solvent can evaporate out of the resin. If the resin gels before the solvent molecules escape, or the application is thick (multiple layers of glass), then the molcules create spaces between the hardener molecules and the resin molecules. This results in fewer molecular links and less polymer chain entanglment. The byproduct is lower heat deflection temperature, less hardness, reduced strength, and reduced stiffness. The solvents will also slow the reaction rate by inhibiting the resin molecules from being close to each other so they can link. The same thing happens in paint if the paint is applied too thickly or the following coats are applied before the solvent has escaped from the previous coats.

If one is happy with the results of a thinned resin then by all means continue.

It's not uncommon for some epoxy combinations to take several days before they can be sanded well. Some epoxy combinations will never sand very well because the hardness of the resin is quite low. The "minute" epoxies just stink for laminating and finishing. Also, if the mix ratio is off, or the 2-parts were not adequatly mixed together, then the cure can be soft and slow. I mix all my resin by weight with a gram scale that goes to .01 grams, and I mix the resin together for about 2 minutes with a squared-off popsicle stick. Is this absolutely neccessary? No. But, if you want a consistent reliable cure like the manufacturer intended then it's worth the effort.



Adam

Props4ever

Here is perfect example of using epoxy resin directly on top of foam structure. Model's main structure is fully skinned with Aeropoxy resin and fairings, fillets, nacelles, presently tail's flying surfaces are being covered with Pacer's Z-Poxy laminating resin...

http://www.rcuniverse.com/forum/m_45...12/key_/tm.htm
Post # 290 n then go to page 47 to see tail surfaces being skinned with Zpoxy n FG cloth...

Samedayservice

Looking good!! start,2006, cured yyet?

kenh3497

Wyo....

I just finished up the bottom side of a couple of wing panels (glass & epoxy over balsa). I got to thinking about this thread and the chart you posted. As stated, I thin my epoxy for the glassing process with denatured alcohol. Your chart says it is the poorest of the three listed to thin with. Even as I stated in a non layup situation where the solvent can evaporate fairly quickly, it seems the acetone would be the better choice over the alcohol.

I have to admit I kind of poo pooed your chart and even the article a bit. Even though I still plan to thin my epoxy for glassing and after rethinking the whole situation, I will try some of the acetone and compare the two results. I guess at one point I was told the denatured alcohol was the only thing to use so of course it became "law" in my mind. The only objection I might have is the evaporation rate will be (I think) higher with the acetone. Probably the reason it is the better product to use.

Ken

wyowindworks

Why not just get a quality laminating resin and not thin it at all? It seems like a compromise in every way. Thinning a bonding resin is going to give a much lower performing matrix than a resin that cures with a higher tensile, compression, and shear modulus. The resin is what causes these "floppy" fabrics to become rigid structures. To say that the resin doesn't matter is a misunderstanding of the function of the resin and how it interacts with the components of the structure. I've tested many resins in my shop (strength, stiffness, heat deflection temperature, etc.) and all things are not equal when it comes to resins. I use and don't use certains resins because of these tests.

If you have been happy with your results then there is no reason to change. Happy is happy. If you want to improve the characteristics of of your structures then you migh consider some alternatives.

kenh3497

I hear what you're saying, I am not fortunate to live in an area where you can run uptown and find anything you need. It is 120 miles to the nearest "good" hobby shop. Yes I know I can mail order but shipping these days is a killer. I would love to use some "better" resins, but I don't use that much at a time and I can't justify buying 1/2 gallon (give or take) and have it go to waist because I didn't use it fast enough.

FWIW, I'm using Bob Smith finishing epoxy because that's all the LHS had. If they would have had some West Systems in a reasonable container size I may have considered that. I didn't really want to spend over $60 for finishing resin. The Bob Smith is much thinner that the traditional glue epoxies. I still like to have it thin as I feel I have much more control over how much I put on the airframe. I don't do the toilet paper thing to soak up the excess as I don't have any excess unless I have a inside corner. Then just a dab with a paper towel in that corner takes care of the problem. The thinned epoxy is not tacky so spreading it around is not an issue. I know you are going to tell me a "quality" resin will cure all my problems. I do not doubt you in any way, shape or form. It basically comes down to what is available, cost and what has worked in the past for me.

I just tried the acetone to thin my last batch of finishing resin. FWIW, I'm using Bob Smith finishing epoxy. The acetone did a fine job of thinning, which I expected. It didn't seem to shorten the amount of working time before the acetone evaporated. But, it did seem to make the epoxy soak in much more that the alcohol did. I only did the bottom of the fuse, so not that much area.

I will be returning to the denatured alcohol as the thinner of choice.

Ken