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  #21  
Old 12-20-2019, 03:49 PM
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jcarne jcarne is offline
 
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Quote:
Originally Posted by snopercod View Post
It's been my experience that epoxy makes a terrible bond to aluminum - even if you prep the aluminum properly by sanding and etching. The Hysol or 2216 makes a much better bond but I wouldn't trust those either without riveting or some kind of mechanical attachment.
I've heard this before but the West Systems I used bonded to sanded aluminum like a beast!
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  #22  
Old 12-20-2019, 06:50 PM
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Default A source for facts on adhesives.

If you want data ( ie. Real experience on RVs), contact HFS. He uses the stuff on his airplanes and does strength testing with his own machine.
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  #23  
Old 12-20-2019, 08:00 PM
TASEsq TASEsq is offline
 
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Default Need a new hammer..

As a data point here is a video showing a hammer test on 2 blobs of epoxy. The first is “dry micro” and the second is flox. The flox is definitely harder than the micro, but the micro was by no means soft. It did dent on the first hit but took a lot of force to split it in half. The flox needed a massive whack - harder than stone.

https://youtu.be/TEAzdL7PTHg

And here is what the “dry micro” consistency was like before I smeared it onto the elevator tip. This was 70g of part A, and 14g of part B, and it took 30 teaspoons of micro to get it to this consistency - was way more than I imagined. It really did have a tendency to “roll up” behind the knife, but if I kept it thick it was ok. In a few hours I will cut it back with a surform then sand tomorrow.

https://youtu.be/3wPJNiNmLyw

Here is the first tip waiting to cure:
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Last edited by TASEsq : 12-20-2019 at 08:09 PM.
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  #24  
Old 12-21-2019, 06:35 AM
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Quote:
Originally Posted by jcarne View Post
I've heard this before but the West Systems I used bonded to sanded aluminum like a beast!
Come to think of it, I never tried bonding West Systems to aluminum. It's good to know it works.
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  #25  
Old 12-21-2019, 06:50 AM
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Default The Joys of Fiberglass

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Originally Posted by TASEsq View Post
And here is what the “dry micro” consistency was like...
Great photos! I did both my wings like that using West Systems and microballoons. I put it on thick then knocked down the high spots with a Sureform file. All the subsequent sanding and filling took two years and just about killed me, though. I had to hire some help lest I go insane.



You can see one area here where I almost sanded through the glass cloth. You don't want to do that.
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  #26  
Old 12-28-2019, 12:39 AM
TASEsq TASEsq is offline
 
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Default Update

So after slathering on the "dry micro" and then taking a lot of it off again with the surform file, i have the following observations:

1. The surform file is a great bit of kit. "Cheese grating" took off 80% of the extra micro and the rest was easy to sand off. In fact, i missed some places and used it after a week or so to see if it still worked on cured expoy - no problems at all.

https://photos.app.goo.gl/GQu4mwpBmq2qiNuM9

2. Once i started sanding, i used 80 grit on a long flat sanding block i have from my model aircraft days - this worked really well and i was able to cut the micro fast back to being flush with the aluminium skin. I was also able to contour the tip to a good shape, taking off almost all of the micro, but leaving the low spots filled. Sands a lot easier than car body filler in my opinion.

Having said this, there were a lot of holes which i would call bigger than just "pinholes" (there were a lot of those). I think the large holes were from voids when i trowelled on the micro - i am thinking it was too thick (you could definitely notice it getting thicker as i worked).

Is it better to make it a little runnier, or try a heat gun to thin it out? Would this "avoid the voids"?
Holes in the counterbalance end skin:

https://photos.app.goo.gl/rhucRqasNz4KMb9U7

Holes near the trailing edges of the tips

https://photos.app.goo.gl/dbkX7xMcm5xra2ze9

I think these are just normal pinholes???

https://photos.app.goo.gl/94s5E5EaScN9wpKZA

What is the best way to fill the larger holes? I am aware that adding more micro is a bad idea - polyester body filler from the car shop? Will the "skim coat" of neat epoxy fill these?

3. I am not a fan of the packing tape. I had 2 layers on the skin and in some spots sanded through either the top or both layers. I used a brand name tape, not cheap stuff, but this still sands through very quickly using 60 grit. This was fine, but meant the stuff was a real pain to remove (kept splitting). I am thinking of going back to the electrical tape.

Next up is the skin coats of epoxy. I plan on the following - with the part off the aircraft:
- Bush some on so that it it all glossy.
- Leave for 30 mins
- Squeegee it all off again with a rubber squeegee
- Leave for 30 mins
- Repeat until it looks like i have built up enough resin to fill the holes that exist.
- hope. pray. et al.

Here is an overall picutre of what it looks like. I was really happy with the hard sharp edge this process had created and a very even gap. I ended up sanding the edge very slightly lower than the skin, to allow for the thickness of the epoxy skim coats to come.

https://photos.app.goo.gl/7sos9FbokpXGF5BX7

Thanks all for following - if anyone has any further advice i would really appreciate it. The shed looked like a Dean Martin song this arvo - snow everywhere.
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Last edited by TASEsq : 12-28-2019 at 12:42 AM.
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  #27  
Old 12-28-2019, 05:46 AM
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snopercod snopercod is offline
 
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Rather than the epoxy skim coat, I would use Polyester Glazing Compound to fill those small pinholes. I think you're going to have to use some more micro to fill that large hole and deal with the resulting discontinuity. Overall, you did a good job.

P.S. I have often thought that it would be great to have a bell jar and vacuum pump to suck all the air bubbles out of the micro before application.
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  #28  
Old 12-28-2019, 07:10 AM
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Quote:
Originally Posted by TASEsq View Post
....there were a lot of holes which i would call bigger than just "pinholes" (there were a lot of those). I think the large holes were from voids when i trowelled on the micro - i am thinking it was too thick (you could definitely notice it getting thicker as i worked).
Is it better to make it a little runnier, or try a heat gun to thin it out? Would this "avoid the voids"?
The divots and pinholes are due to air entrained in the dry micro while mixing. A really dry mix will hold every bubble, while a wet mix will allow the bubbles to float to the top. I would not go wet just to reduce the number of pinholes in the finished product, as dry micro is lighter and easier to sand.

A different mixing method may entrain less air, but mostly it's something to ignore. All the little stuff will be eliminated in the sealing stage.

Quote:
What is the best way to fill the larger holes? I am aware that adding more micro is a bad idea - polyester body filler from the car shop? Will the "skim coat" of neat epoxy fill these?
The big divot is a case of "whatever works"...a dab of micro or two part polyester prior to sealing being the most common. I'd simply begin the seal coat process by wiping some dry micro into the divot with no excess (surface slightly low), and skim neat epoxy over it just like the rest, no waiting.

A word about adding additional layers of micro or body filler products. The caution is about adding patches of a material with a different sanding density, as it's very hard to sand a surface optically flat when part of the surface is soft, and another part is hard. The final painted surface will be wavy, something which becomes apparent after paint. The best bodywork reflects a perfect image across a large panel...no optical distortion. It takes a lot of blocking to achieve that finish.

Doesn't mean you can't add filler. Simply avoid patchwork filler applications in areas where being visibly flat will matter. I have seen unpainted RVs where the builder tried every filler product in the body shop catalog. Sorta looked like a spotted dog in multiple shades of white and gray and red and beige. Those surfaces will be wavy when painted, but given the right color and scheme, it won't be noticeable at 10 yards. RVs are mostly made up of wavy sheet metal anyway. Next trip to OSH, go search out a past Grand Champion composite plansbuilt, and sight lengthwise down the big surfaces.

In recent years my favorite epoxy skim application method has become cheap foam rollers. First coat gets squeegeed around some to fill the holes, then rolled to even the surface. All additional coats are rolled on.
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  #29  
Old 12-28-2019, 12:50 PM
TASEsq TASEsq is offline
 
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Thank you Dan - appreciate your prompt response!

I’ll mix up my batch of neat epoxy to do the skim coat and remove a bit and mix up some dry micro, full the super large holes then do the skim coat straight away.

So to clarify your process - you apply the epoxy with, say, a brush initially, squeegee it off, use a roller to add texture and remove the squeegee marks, then from there the additional coats are just rollered on? (Not squeegeed off again and roller for texture etc?)
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  #30  
Old 12-28-2019, 01:46 PM
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Precisely.
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