Cory Bird's Raw Epoxy Finishing Technique

[Wayne Hicks]

I recently read about the finishing technique that Cory Bird used on his OSH grand champion Symmetry. The technique is dry micro, contour to 36 grit (yes, 36 grit), then skim coat and squeegee up to 5 successive coats of pure epoxy (yes, pure epoxy) to fill all pin holes and scratches. Curious, I tried a variant of it this weekend on the bottom of my wing. 4 pinholes TOTAL! Except for two small low spots near the aileron spars, I was almost ready for paint on the very first try. I am so psyched I can hardly stand it. If I was starting from scratch I would definitely give the full technique a try!

 

I was already contoured to 120 grit on my wings when I came across the article, so I didn’t use that many coats. In my "test case", I applied the pure WEST with a roller, waited 10 minutes, the squeegeed it all off. The pinholes were almost filled. So I applied another layer, this time lightly squeegeeing to leave a very thin layer. I probably left on too much WEST. I need to find the balance between scraping it all off and leaving barely enough on.

 

Anyway, I started sanding off the WEST. True, it was a little hard to sand off. But a MIRACULOUS thing occurred. Almost all the pinholes and large scratches got filled. And I'm talking about very big pinholes, like the size of small peas. Even more miraculous, because the WEST is so tough, my 3-foot sanding board (100 grit, then 120 grit) "cut" the tops off the most inconspicuous of high spots that my first contour to 120 never caught! I continued to sand, sand, sand until all the WEST was almost, almost, almost sanded away.

 

Steve (an exceptional hangar mate if ever there was one) sprayed the wing with primer. Like I said above, I sanded it off and was rewarded with a wing bottom that had only 4 pinholes TOTAL. For test comparison, I didn't WEST the lower winglet. Just sprayed primer over it. Of course, it was loaded with pin holes.

 

So what does this all mean? Since this is the first time I've ever filled, sanded, and primered ANYTHING, I have no basis for comparison. But I can vouch that skimming with WEST really does fill scratches and pinholes of all sizes. And if you're an aggressive sander like me (meaning, I have no patience, no finesse!), the hard coat left by the WEST helped me to achieve an even better contoured finish. The other obvious benefit is leaving the tiniest of WEST layer helps to harden the micro. I've heard others say that this helps prevent print-through later on in life.

[Jon Matcho]

Great contribution Wayne! I'm getting motivated just reading about it. It must be great that you see the light much brighter where you're at in the tunnel.

 

My only question is why it took Cory upwards of 10 years to finish his plane from first mention? http://www.ez.org/cp77-p4.htm

[Jon Matcho]

I'll answer my own question... "Because that's what it takes to win Grand Champion Extraordinaire!!!"

 

Still, sounds like a GREAT technique for finishing and one to bookmark.

[Wayne Hicks]

I guarantee it didn't take Cory 10 years of 8-hour days to build his plane. I bet he's like some of us who work on our planes when we can in between taking care of higher priority things -- like life. In the Sport Av article, he did say he rebuilt some parts of the plane more than once.

 

For me, my builder logs say that I've worked on my plane < 1 hour/day average. That's pathetic. But the plane comes last -- behind the wife, Bible study, career, handbells, and honey-do's.

[Nathan Gifford]

All this sounds neat, however:

Why not use MGS instead of West? You wouldn't need to change the ratio of your epoxy pump.

 

Is the difference in hardness (how much is it anyway) that significant in this application?Though most of these parts go through very little flexing, in the long-term having a slightly more flexible epoxy be a good thing?

 

[Wayne Hicks]

(Ever feel like this is a two-site conversation? Like...double-talk? )

 

Why not use MGS instead of West?

 

--> The Cory Bird article (http://www.ez.org/cp77-p4.htm) recommends you use the same epoxy used to build the airframe…if you can. The caution is to use an epoxy that can cure when applied very thin. Some structural epoxies will not cure when applied thin. They stay gummy. I have no idea if MGS will cure thin. I’m assuming it does. The WEST was made to cure thin, so that’s what I’m using.

 

---> When using WEST, you usually buy the WEST plunger pumps specifically made for the WEST cans. No need to change the ratio on you structural epoxy pumps.

 

Is the difference in hardness?

 

--> Yes! Some epoxies cure very hard in one day. MGS and EZ-Poxy to name two. Others, like WEST and Aeropoxy, take a few days to reach hard cure. You will have a tougher time sanding MGS and EZ-Poxy than you will with WEST. That’s one of the advantages to using WEST and as to why it’s been the popular Chapter 25 choice all these years. However, wait a week or two with WEST and you’ll be wishing you’d have sanded it off sooner. Ask me how I know….

 

in the long-term having a slightly more flexible epoxy be a good thing?

 

--> In looking at canard airplanes the past 8 years, the only cracking I’ve seen is in large thicknesses of micro (which doesn’t like to be flexed) and at the interface of ill-fitting parts.

[Jon Matcho]

The caution is to use an epoxy that can cure when applied very thin. Some structural epoxies will not cure when applied thin. They stay gummy. I have no idea if MGS will cure thin. I’m assuming it does.

I never knew that. When I did my seatback, the BID overlap/seam had a rough edge so I decided to paint it with some MGS 285. The seam remained gummy for months, but finally cured. HOWEVER, my cup was also NOT fully cured (failed scratch test) so I had assumed I'd mixed an 'off' batch (the cup eventually cured too).

 

Given that the cup remained gummy, I don't think this was because the MGS was spread too thin. Another builder told me he frequently paints MGS on seams, so I'd have to believe that MGS does indeed cure when (a) mixed properly, and (b) spread thin.

 

I'll test this next chance I get.

[Jerry Schneider]

In my experince, MGS has had no problem hardening if spread thin. Alphapoxy, which I use for finishing does stay gummy though.

 

FWIW

[Jon Matcho]

In my experince, MGS has had no problem hardening if spread thin.

That's good to hear -- I must have mixed a bad batch.

 

...I use for finishing...

Woah... getting closer, eh Jerry? I'll be in town 11/10-11/13 and will have to sneak a visit out to see your project again. Feel free to save some 2-person work if you need.

[mtmcquee]

What do you guys think in terms of weight penalty (epoxy), with Corys' technique?

 

In terms of any entire aircraft constructed this way?

 

Travis McQueen

[argoldman]

That's good to hear -- I must have mixed a bad batch.

 

 

MGS, if dropped on a plastic table protector will harden to a knife edge.

 

If your batch was soft, look at the porportions and mixing technique. The fact that it takes days to set is not a positive, as you have no idea of what is happening below the cured surface. This is not polyester and will not totally cure to proper strength if the ratios or mixing is off. (polyester will)

 

Keep mixing

[Nathan Gifford]

...In terms of any entire aircraft constructed this way?...

This is only a finishing technique. You should not use to build the laminating layers.

 

Somebody correct me where I am wrong, but by adding micro you are making the epoxy lighter. I would think you would want this in the laminate since it would probably lead to a weaker bond.

[Wayne Hicks]

Chapter 25 filling and sanding consists of several steps

(1) Rough contour -- this is the dry micro

(2) Filler -- this is high build, not to be confused with dry micro

(3) Final prime

(4) Top Coat paint

 

Per Cory Bird's article, a gallon of pure epoxy weighs less than high build primer, which contains lots of solids in it. So once he's got the dry micro rough contoured to 36 grit, he then goes into Step 2 and fills the scratches and pinholes with raw epoxy. Most everyone else takes the dry micro to 80-100 grit, then goes to high build primer.

[SammyQ2]

I tried using this technique and I really liked.

 

In my Q-200 rebuild project I did track how much West 105/206 I added to the fuselage. It came to about 2.2 lbs.

 

You can read all about it here:

http://samhoskins.blogspot.com/2008/08/doing-epoxy-wipe.html

 

Sam Hoskins Q-200

Murphysboro, IL

[jpolenek]

contour to 36 grit (yes, 36 grit)

I'm planning to use this technique on a surface which I have filled & sanded to the shape I want, and it is quite smooth (maybe 120 grit or better). If I try to rough it up with coarse grit at this point, the sandpaper will take more out of the filled areas than the glass areas, and that would change the shape.

 

Is the 36 grit finish intended to avoid unnecessary fine sanding since the epoxy will fill the grooves anyway, or is it required in order to provide more surface area for improved adhesion?

 

Joe Polenek

[Lynn Erickson]

I'm planning to use this technique on a surface which I have filled & sanded to the shape I want, and it is quite smooth (maybe 120 grit or better). If I try to rough it up with coarse grit at this point, the sandpaper will take more out of the filled areas than the glass areas, and that would change the shape.

 

Is the 36 grit finish intended to avoid unnecessary fine sanding since the epoxy will fill the grooves anyway, or is it required in order to provide more surface area for improved adhesion?

 

Joe Polenek

the 36 grit finish intended to avoid unnecessary fine sanding since the epoxy will fill the grooves anyway

[Kent Ashton]

I'm planning to use this technique on a surface which I have filled & sanded to the shape I want, and it is quite smooth (maybe 120 grit or better). If I try to rough it up with coarse grit at this point, the sandpaper will take more out of the filled areas than the glass areas, and that would change the shape.

It's way more work to finish a 36-grit surface rather than something like 320-grit surface. Epoxy will stick just as well to either one but 36-G scratches are the Grand Canyon of scratches. Even 120-G sounds too coarse to me as a final surface before to starting to fill pinholes. I'd try to get it to at least 220-G if you can do that without sanding into the glass. For my next paint job, I will try to get to a 320-G surface before going to the straight epoxy pinhole filling.

[Lynn Erickson]

It's way more work to finish a 36-grit surface rather than something like 320-grit surface. Epoxy will stick just as well to either one but 36-G scratches are the Grand Canyon of scratches. Even 120-G sounds too coarse to me as a final surface before to starting to fill pinholes. I'd try to get it to at least 220-G if you can do that without sanding into the glass. For my next paint job, I will try to get to a 320-G surface before going to the straight epoxy pinhole filling.

thats a waste of time to sand the filler more than 60 grit. The epoxy is to fill the pinholes and then use high build primer to fill everything else. actually the epoxy is also a waste of time the new epoxy high build primers are the way to go. fills the 36 grit and you end up with no pinholes. we did a Lancair IV with a carbon fuselage this way and did not have any pinholes

[argoldman]

thats a waste of time to sand the filler more than 60 grit. The epoxy is to fill the pinholes and then use high build primer to fill everything else. actually the epoxy is also a waste of time the new epoxy high build primers are the way to go. fills the 36 grit and you end up with no pinholes. we did a Lancair IV with a carbon fuselage this way and did not have any pinholes

Agreed, I've been doing 40 grit and then a top and/or fill layer of Aeropoxy light, down to finer.

 

What high build epoxy primer are you recommending???

[Kent Ashton]

thats a waste of time to sand the filler more than 60 grit. The epoxy is to fill the pinholes and then use high build primer to fill everything else.

Sanding is sanding. Either "waste time" sanding filler or waste time sanding high-build primer.

 

Sanding the filled surface from 40G to 320G also requires less high-build primer and results in a lighter airplane.

 

What's the logic in purposely leaving deep scratches in a surface so you have to use high-build primer to fill them?

[argoldman]

Sanding is sanding. Either "waste time" sanding filler or waste time sanding high-build primer.

 

Sanding the filled surface from 40G to 320G also requires less high-build primer and results in a lighter airplane.

 

What's the logic in purposely leaving deep scratches in a surface so you have to use high-build primer to fill them?

It boils down to time spent sanding . Shaping and sanding with 40G is very rapid and takes relatively little effort. After the 40 is done, a coat of aeropoxy light, using the high points of the prior sanding merely the areas on each side of the "scratch left by the 40 grit, just filling in the scratches requires very little post application sanding with 8o then finer.

 

Don't know how much added weight that adds, --probably none-- since if you build up, sand down then sand finer and finer, you end up with the same surface, It's just a matter of building it from the inside rather than sanding it away from the outside. For what it's worth, this technique served me well on my dragonfly and the wings of my cozy (still waiting for the final coat and sanding). "you say either and I say Ether":cool:

[Lynn Erickson]

Sanding is sanding. Either "waste time" sanding filler or waste time sanding high-build primer.

 

Sanding the filled surface from 40G to 320G also requires less high-build primer and results in a lighter airplane.

 

What's the logic in purposely leaving deep scratches in a surface so you have to use high-build primer to fill them?

first I said to sand to 60 grit not 40. 60 grit scratches are not very deep and will fill with one coat of high build.

second high build primer does not weigh any more than micro

And who said anything about leaving deep scratches in the surface.

If you try to sand the micro to 320 you will have a very hard time keeping the surface true. the soft micro will sand away much faster then the hard glass and you will leave a wavy surface. sand the micro with the course paper until you start to see glass and stop. fill any low spots and repeat. when all the low spots are gone, apply high build and sand to 220. then prime and paint.

[Lifessamsara]

So Lynn, you don't subscribe to the Cory Bird method at any stage of the finishing process?

[Lynn Erickson]

So Lynn, you don't subscribe to the Cory Bird method at any stage of the finishing process?

no. it is just not necessary with the new high build epoxy primers. the problem with the method is that the surface needs to be really true before you put on the raw epoxy. if it is not true and you will not know until you prime and then what do you do ? add more filler or try to sand through the coated filler and then coat it again. I developed a similar process long before Cory was born and it does work. used it on my canard and then did the rest of the plane with high build. the new stuff just cuts out the extra step and it sands a lot faster with no pinholes. the stuff is designed to go over molded carbon/epoxy parts and you can't have more pin holes than that.

[ErlendM]

the new stuff just cuts out the extra step and it sands a lot faster with no pinholes. the stuff is designed to go over molded carbon/epoxy parts and you can't have more pin holes than that.

Lynn, what stuff are you talking about here? Do you have any product-name to share?