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Old Aug 27, 2012, 08:25 PM
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Bountiful, Utah
Joined Dec 2003
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How do you make a hard smooth fillet that will take epoxy paint with good adhesion?

The picture is my sailplane before its maiden flight showing the intersection of the wing to the fuse. I used vinyl tape bridging the wing root and fuselage to keep the wing from sliding off or moving. A steel dowel passes through the fuse into the wings. The paint lifted (with primer) along the narrow edge between the wing and the top hatch when removing the tape.

The slightly rounded top hatch is balsa sanded to shape and primed and painted with 2 part epoxy paint. No problem there, the paint stuck and did not lift off. But the narrow edge between the wing root and hatch was slightly built up with lightweight speckling (Red Devil lightweight spackling) to make the narrow fuse edge flush with the access hatch and wing root. The paint and primer lifted in spots after removing the tape and I suspect that the lightweight spackling is the culprit because it dents easily and is soft under the paint. HELP!

I will now remove the spackling and paint down to the bare wood. What should I use to replace the speckling that I can sand and shape that will:
1. stick well to wood
2. dry or cure very hard
3. is somewhat sandable so that I can shape it to make it flush with access hatch and wing root.
4. will accept epoxy (2 part Klass Kote epoxy paint) with good adhesion?

I have heard that bondo (used in automotive body work) does not adhere well to wood. I'm sure there are at least a couple of good alternatives and I hope you guys can help me. Thanks in advance.

Dave
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Last edited by dwmumford; Sep 26, 2012 at 07:56 PM. Reason: typos
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Old Sep 25, 2012, 08:51 AM
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Joined Jul 2010
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The cheapest way (but not necessarily the lightest) to achieve the filleting you want is to mix microballoons with 30 min epoxy to the consistency of peanut butter. Spread that into place and after it cures you can rough and fine sand it to the shape you need. I've never had any problems with any kind of paint including dope, enamels, poly, or two-part epoxy sticking well to it.

For larger fillets where there must be significant build-up (such as wing saddles) I use this product:

http://www.aircraftspruce.com/catalo...s/superfil.php

It's not cheap but you get enough to do a lot of models and it appears to have a good shelf life. It's very light and easy to work with. It seems to stick well to balsa but you may want to put a thin coat of finishing resin down first just to harden the balsa a bit and ensure adhesion for the superfil. The superfil sands and finishes nicely and every paint I've tried sticks well.

Curt
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Old Sep 26, 2012, 07:53 PM
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Bountiful, Utah
Joined Dec 2003
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thanks for the tip Curt. After reading and doing some experimentation I have decided that the micro balloons and finishing epoxy resin works very well because it adheres well after you first put down a thin layer of straight finishing resin or epoxy and then when it starts to cure mix some micro balloons with finishing resin. When cured, start sanding. Also it cures rock hard. I tried it on a few pieces of balsa and pine. It works well. I may try your Superfil later, but for now the other method works nicely. thanks
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Old Oct 04, 2012, 09:49 PM
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637 Posts
I had precisely the same issue with my old Aquila, I replaced the spackling with Bondo that I thinned with denatured alcohol which does not effect the cure but allows a near perfect application and little or no sanding.
Problem solved.
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Old Oct 04, 2012, 11:22 PM
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Bountiful, Utah
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Quote:
Originally Posted by jetmaven View Post
I had precisely the same issue with my old Aquila, I replaced the spackling with Bondo that I thinned with denatured alcohol which does not effect the cure but allows a near perfect application and little or no sanding.
Problem solved.
JETMAVEN, that seems like a good solution where small areas are concerned, as long as you don't have to use a lot of the Bondo. But I also found that the mocro balloons with finishing resin works well too.
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Old Oct 05, 2012, 11:19 AM
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Dip your finger in alcohol and you can smooth and shape the epoxy mixture to eliminate much of the heavy sanding.
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Old Nov 19, 2012, 11:14 AM
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United States, NY, Lewiston
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I harden spackle filets with thin CA glue after I get them sanded exactly how I want.
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Old Nov 22, 2012, 12:23 AM
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ADAGIO (slow dancin'man)

The paint lifted (with primer) along the narrow edge between the wing and the top hatch when removing the tape.

The paint and primer lifted in spots after removing the tape and I suspect that the lightweight spackling is the culprit because it dents easily and is soft under the paint. HELP!

I will now remove the spackling and paint down to the bare wood. What should I use to replace the speckling that I can sand and shape that will:
1. stick well to wood
2. dry or cure very hard
3. is somewhat sandable so that I can shape it to make it flush with access hatch and wing root.
4. will accept epoxy (2 part Klass Kote epoxy paint) with good adhesion?

I have heard that bondo (used in automotive body work) does not adhere well to wood. I'm sure there are at least a couple of good alternatives and I hope you guys can help me. Thanks in advance.

Dave[/QUOTE]

Dave;

It's been a while since I flew, but one of the methods somebody else showed me has been very useful in lightweight structure in boats which Úven stands up to a bit of rubbin is 'racin'.

In Australia, we call what you call bondo bog as it looks like a blob of something unimaginable. If bondo sets as hard as bog can, I can see your problem. There is no way it will adhere properly (as in make a structurally sound joint) to balsa and stay there under load - the balse grain simply lets go as it has no useful structural integrity.

There is a really much easier and far more stable lightweight solution (I reckon). Solution is the solution.

Balsa is excellent with this, but lightweight plaster wall spackle is even better.

Shape the balsa and wipe it over with a DAMP cloth, wait for the fuzz to appear, rub it down again and, while the balsa is still damp, pour superglue over it. Sets up really hard and sands a treat. If the balsa is a bit thick, prick it with a skewer or thick pin first to both aid adhesion and to penetrate forming hard pins of set superglue. It adds hardly only grams in weight and gives a really good paint finish without wood grain to fill.

When it has set, start sanding and paint preparation. Be pepared, it does get hot during set, and do it outside as it stinks, and it can be hard to sand, but the strong smooth surface is fantastic. This technique works on any porous surface and epoxy sticks to it like, well, glue. The poxy will needs a roughened surface for a mechanical grip. Fantastic stuff, and all easy to do.

Another superglue tip: make up and join with carpenter's wood glue and smear superglue over the joint. Goes off like a shovel full of prawns in the sun Oz for really quick.

Enjoy life a bit more.

I'm not certain about iron on covering but the stuff should stick to a nice smooth surface.

SIAS - Oz for suck it and see.
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