I have some microballoons, however, I have never used them so I will try on some scrap wood first. Question: What is the best "mixture" of what type of glue and how many microballoons for fixes involving 1/4 inch etc?

Mix the epoxy FIRST then mix in the microballoons. It sometimes does not set if you do it the other way round.

1/4 inch ?? If you are talking about pits then it would be OK but if they are holes scarf in new wood.

I don't use micro balloons for repair, only for cosmetic uses like wing fillets, etc. You give up some epoxy strength when you add micro balloons to the mix.

I have to agree that micro balloons should not be used for repair. If you think about it what you are doing is filling the epoxy with air space and the air space has no strength. Best save them for cosmetic uses as Jim Dines said.

Likewise.

I don't understand why the Composite-ARF directions call for Micro-Balloons for all of the heavy load bearing construction of their planes.

I see a lot of my club members following the directions exactly, and I've always speculated that they are taking a risk by doing this.

Anyone have a suggestion for a better "filler" to mix in low quantity with Epoxy?

Fiberglass chopped up into bits knits the epoxy together pretty good. For ding repair on wood, lightweight spackle is good, but takes forever to dry if it's too deep. Bondo has a "Deluxe" formula that sands pretty easily and is good for feathering edges, like where a layer of a FG patch ends. Microballoons mix well with all types of epoxy and the drier the mix, the easier it will be to sand. If you leave the mix runny, it is very self leveling and works well like that, too.

What about wood sawdust?

Using spackle worries me, because even when dry it is not a strong material and tends to fragment easily.

Spackle works fine, it's the best filler over raw foam because it sands at about the same rate as foam or balsa. Adds very little weight.
I've never tried balsa sawdust with epoxy.
Balsa sawdust and CA sand at the same rate as concrete.
If spackle is encapsulated with epoxy, paint, BalsaRite, etc., it holds up OK under cloth or film. It is a ding filler, not for structural repairs.

I use the spackling for a weave filler when I lay fiberglass down the wing center. I put it on with a credit card so that I have a very thin film. Let it dry and sand with 220 and the only spackle that you have left it between the fibers. I also use it to blend the fiberglass to the balsa. Again nothing structural.

Atomized steel and aluminum are used as epoxy fillers as is carbon fiber in certain weapons applications. The filler might interfere with our radios however, I've never tried it on a plane.

Man, I'd shy away from metal filler. I use the stuff bedding rifle barrels to the rifle stock. It's mixed into epoxy just like microballoons is mixed in for modeling. And the mix is HEAVY. Metal gives epoxy an impact strength. And we really don't need that. Unless you normally crash so hard nothing remains, and you want SOMETHING to remain.

In practice, a microballoon mix can be quite strong. If you need it to be, do what combatpigg says and mix in some fiberglass flox along with the microballoons. It's really not hard to do once you've done it some. "Once you've done it some" is the hard part.

There happens to be a "microballoons" made to add strength and lighten up the resulting mix. If I remember correctly, it has "phenolic" in the name. I'd run down to the shop and check, but done that a couple of times already. I think Sig sells it.

So the best microballoons mixture for medium repair would be that stuff if strength matters. But someone mentioned scarfing in balsa, and that'd be the best to do and then use the mix to glue it. It'd glue and fill both, allowing the filler wood fit to be less than perfect. Save time that way.

Fiberglass sounds like a good idea, particularly for load bearing areas.

I haven't tried the particular microballoons you mentioned.

However with the standard stuff, it seems to me that the filled area is somewhat weaker than using straight epoxy.

Let me restate. For cosmetic only. Speaking of. Which one is better? microballoons or hobbylite wood filler?

Better?
Fillers are almost always no mixing. Epoxy, you mix, then mix in microballoons, then try to get it on before it kicks. If better = easier, there ain't no contest.

On the other hand, if you're trying to fill a large gap the epoxy/microballoons can do it and won't crack or fall out later.

Filler loses the contest if the holes are much larger than pinholes. But it is going to be lighter. Trying to mix epoxy/mb to match filler's lightness is really difficult.

thanks Darock,
Used hobby lite balsa filler for a nice finished look. Start on building wings today.

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I often use both the phenolic and the Microballoons . Properly mixed, both are quite strong with the Microballoons (I believe these are fiberglass) being somewhat stronger. Always use a slow epoxy, 30 minute or longer, and be sure to mix the two parts well before adding the balloons. Keep adding balloons until you get a mixture like very stiff peanut butter so that it will not run or sag after application. Either will sand quite easily, at least easy as compared to fiberglass or epoxy alone. While still uncurred, you can smooth it with your finger wetted with alcohol to minimize any sanding requirements. If you area covering with a light colored covering use the Microballoons as the phenolic are a dark brown and will show through most coverings while the Microballoons are a nice white and easily covered.

I recommend you visit the West Systems and System Three websites to check out the available epoxy additives and their uses. I keep microballoons, flocking (milled fiberglass), wood flour (fine sawdust), calloidal silica (a "thixotropic" thickening agent - like ketchup it stays put when placed somewhere) and various glass and fiber matting. Things like new coffee filters make a great repair laminate with epoxy.


http://www.westsystem.com/ss/how-to-use/

http://www.systemthree.com/members/m_index.asp (The Epoxy Book in pdf is a free download and very worth reading through. Geared towards boatbuilders but the same facts apply to small batches for R/C)

Some additives bulk up the epoxy for filling gaps, some strengthen it, some change the properties (for better or worse) and they can be abused to make the adhering properties worse. Add too many microballoons to a large amount of epoxy and you may have a melt-down thermal reaction.

As mentioned above: always mix the epoxy first. Let it stand briefly to link and then add the filler(s).

USE FLOX

Yeah, I'm looking at strength however, hence my reference to the Composite-ARF instructions which use microballoons for heavy load bearing areas, something that I find strange and disagree with.

hi thanks for this thread never did know how to use microballons correctly till now enjoy TONY

Great stuff, very informative.

This raises another question....

From that literature it appears that the cure time is determined entirely by the hardener, not the resin.

But what happens when you mix epoxy mixtures from different manufacturers?


I had an extra bottle of Great Planes hardener that I wanted to use with some Hobbico Resin.

I tested this mix, but the result did not look right.

I've never tried mixing them. Some are 1:1, some are other ratios (i.e. 60/40, 90/10). Those that are 1:1 probably have other compounds in the hardener (certainly more than just one ingredient) and different manufacturers may have differing ideas on which to include with the epoxy and which with the hardener.