Cory Bird's Raw Epoxy Finishing Technique

[Dust]

OK, makes sense to me now, in woodworking, when you fill the grain with grain filler or finish for that matter, you have to sand very carefully to not open the grain again, this process sounds the same.

i can handle that - thanks for the info

[Wayne Hicks]

In woodworking, when you fill the grain...you have to sand very carefully to not open the grain again.

----> The West skim coat cures hard. But this is a GOOD THING! It does take more than a few passes to intentionally penetrate through the west. You won't disturb your micro contour unless you really want to. So what happens is you only end up contouring this very thin skim coat. About 20 minutes' work! Unbelievable. It doesn't have to be perfect either. The skim coat is thinner than a coating of high build primer.

---> The Cory article never mentions using primer. I'm thinking he does. I'm thinking that when the article says, go straight to final paint, that it means go straight to final painting system. That would be final primer + top coat paint.

[tnt]

Thanks for bringing in this info Wayne. I'm looking forward to using it.

[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]

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.

[Waiter]

In the early days, The main reason for using West for the finishing was that you could use the "Fast" hardener. This was not an option with EZ-Poxy and Safe-T-Poxy. By using the fast hardener, you could do several fill/sand iterations in a single day. If you used EZ-Poxy or Safe-T-Poxy epoxy, you would usually have to wait a day or two for the epoxy to cure before you could sand, as they didn't offer a FAST hardener.

MGS, or whatever the Plans call for should be OK for the fill/sand finishing operations. If they offer a fast hardener, then you may want to use it if you would like to fill/sand several times during a work session.

Waiter

[Wayne Hicks]

I might need to do offer a different opinion from my friend Waiter. While you may use FAST when curing the micro, I'm sure you shouldn't use FAST anything for raw epoxy finishing. Pure WEST with 205 fast hardener will exotherm on you in a hurry! Especially if you have it exposed to the sun. It will smoke, cook on the surface, and bubble up. It may/will create enough heat to melt the foam under the skins.

Besides, FAST will kick off and become too gummy before you can coat the wing and squeegee it off. (9-12 minutes at 72 degrees. Quicker at higher temps.)

I understand that RAF never approved the WEST/205 hardener combination because of the exotherm and its potential heat damage to our foams.

One of our hangar mates is a retired A&P for United Express. He used this skim coat technique to prepare the 15-layered kevlar leading edges for accepting new pneumatic de-icing boots. He mistakenly used 205 one day instead of 206. He ended up having to replace the kevlar.

I wouldn't use FAST for raw.

[Waiter]

RGR on the exotherm.
I can't speak about the "raw epoxy finishing" method, because I've never tried it, but West will exotherm even with the micro. You want to mix it, and get it on the surface ASAP. On warm days (80 - 90deg F), it will be cured and ready to sand in 30 minutes.

[Dust]

Welllllll, I've already bought lots of mgs fast - i'll play with the ratio and see ifin i can get it to work with this system

[neverquit]

I've heard it's not a good idea to mix epoxy brands. Is that only when in liquid? Can I spread West over MGS?

[Dust]

I would do it - once the epoxy is cured it does not seem as it would care which brand is applied over it

[Wayne Hicks]

I emailed Cory Bird and learned more about his raw epoxy finishing technique. The CP article left the impression that he went from epoxy to final paint and skipped the high build priming and primer/sealer coats.

Not so. The only reason he skim coats the wings (Cory calls it a "resin wipe") is to get rid of pin holes and sanding scratches.

What he does is:

1. Sands the dry micro with 36 grit to within 0.020 inches of perfect contour.
2. Applies 5 skim coats, lets cure, then wet sands with 150 grit.
3. Applies a PPG DP-48 primer/sealer coat and blocks that out with 220 grit wet sand. (Repeated until he obtains an optically perfect surface finish.)
4. Final primer/sealer coat is blocked out with 350 grit wet sand.
5. Then final top coat paint.

So that puts his process more in line with what we already do per Chapter 25. Except, time is saved by skipping some of the contour sanding cycles with 80 and 100 grit. It is far easier and less time consuming getting primer into perfect contour than trying to do the same with micro.

All of this and other big ideas are being included in my Chapter 25 updates to my website. Just go to my home page (click below), then "Chapter Summaries", then "Chapter 25."

[Nathan Gifford]

Originally Posted by neverquit

I've heard it's not a good idea to mix epoxy brands. Is that only when in liquid? Can I spread West over MGS?

Correct me if I am wrong, but using epoxy in the manner described is not structural. Maybe this use is less demanding?

[Dust]

Thanks Wayne - am about to try this. did try something else last week - put mostly fast in the pump - microed and then in about 2 hours - microed again - got the build i was unable to get when i was trying to fill and contour the canards a few years ago

[Jon Matcho]

Originally Posted by neverquit

I've heard it's not a good idea to mix epoxy brands. Is that only when in liquid? Can I spread West over MGS?

Never mix resins or hardners from different systems/brands. HOWEVER, I am now certain that it is fine to interchange different approved epoxy systems (those meeting the required specifications) once the epoxy has fully cured.

I asked the Glue Guru himself. Read more here if you'd like: http://www.canardzone.com/forum/showthread.php?t=1343