Cory Bird's Raw Epoxy Finishing Technique

[Wayne Hicks]

Dust:

I'm convinced about using the WEST skim coats. DEFINITELY filled all my pinholes and scratches. But I could not bring myself to do what Cary did and stop at 36 grit. The 3M paper I use is like dragging boulders across the micro. Sure takes down the micro in a hurry, but the scratches are deep!

I jumped to 80 grit and found that to be a more appropriate stopping point. The 5 skim coats covered that well.

There's no need to progress past 80. I know this because I was using 120 for a while. All that did was polish the surface and open up finer and finer pinholes. That's bad because there comes a point when the pinhole becomes too small for the skim coating to work. Too small, the raw epoxy won't flow into the pinhole.

[Dust]

hear you on the 80 grit

just why do you feel it must be west system?

[Wayne Hicks]

You're right, I should say "epoxy" instead of generically using the word "WEST".

Skim coating can be done with any epoxy as long as it can cure when spread thin. But some epoxies won't cure when spread thin. They stay gummy. That would be bad.

Why WEST? Old habit I guess. It's the tried and true epoxy mentioned in the plans, it's been in use by canard builders for a long time, and it cures hard even when spread thin.

[Dust]

OK, i just have an attitude on west - built some christmas gifts and the glue failed - called goughen bros and talked to techs - they said that all planer parks must be sanded from the wood as the planer compresses the fibers on hard maple stopping penetration.

Here i am the furniture maker and cant make a simple glue joint - pissed me off

msg oops mgs hardens just fine in thin film

[Nathan Gifford]

From what I read so far I wonder if there is some kind of consensus building?

Epoxies
Are we saying that MGS/West will work well in this application?

That either West or MGS can be used on either products' CURED surface?

Does West give the builder more time to sand the surface than MGS?

Either MGS product can be used (285/335) but it must be sanded more quickly than West Systems will require?

Sanding
Epoxies should be sanded with 36 grit and then 80. You should not use finer grits at this point since you may get more pin holes and later steps will give you a good result.

[Wayne Hicks]

A friend used WEST to glue Apache helicopter blades to test fixtures for destructive testing. The blade failed, the WEST joints didn't.

A friend built an 18-square meter sailboat from WEST. It's never broken either.

I've ALWAYS had good luck using WEST. I adore the stuff.

[Wayne Hicks]

Nathan:

You're making me confused!

This "skim coating" that Dust and I keep bantoring about is for Chapter 25...ONLY! That's the filling, sanding, and contouring of the airframe AFTER it is built. Skim coating is only for filling the pinholes that occur in the filler material (the dry micro) that's applied over the cured fiberglass on the completed airframe. (Please see my chapter 25 web pages.)

Skim coating is NOT for filling pinholes in the fiberglass itself. (Although I must say that if you're getting pinholes in your fiberglass layups, you're not wetting out the cloth enough.)

For all the other chapters 3-24, use your ONE structural epoxy of choice to fabricate the airframe components. Use whatever grit you want to prep-sand any area that requires another layup to applied to it. (I prefer 36 grit.) Skim coating here does no go, only adds weight.

As for epoxy compatibility:
it is my understanding that the rule of thumb is:
(1) Brand A Epoxy will stick to Brand B epoxy.
(2) Epoxies will stick to polyesters, but polyester will not stick to epoxies.
West will stick hard onto MGS, MGS will cure hard to WEST.

As for sandability (of dry micro):
Dry micro made from WEST is soft-sandable for about 4 days. John Slade and others report the same soft-sandability from dry micro made from MGS. But let's not kid ourselves. "Soft-sandable" is a misnomer, because they all cure hard and are like sanding concrete.

As for duration of sandability (of dry micro):
I've known people who've had dry micro on their airframes for years before ever going back and contouring the part. You can still sand it. Dry micro will never become so hard as to not be sandable. The sanding difference between fresh dry micro and old dry micro is probably not that noticeable anyway.

As for sanding schedule:
You made the statement, "Epoxies should be sanded with 36 grit and then 80. You should not use finer grits at this point since you may get more pin holes and later steps will give you a good result." Again, my discussion about this 36-80 grit sanding schedule only applies to sanding off the dry micro during the contouring process in chapter 25. I do not mean to imply that you should be sanding the raw fiberglass with 36, then 80 grit during construction.

[Dust]

Originally Posted by CBarber

Chris
on the Velocity Chapter that deals with finishing...micro (used as a verb), sand, repeat, repeat, repeat (from the Department of Redundancy Department). Heh, the classics still crack me up

ok chris - this is my experience, but i think the repeat and repeat and repeat is the killer - last week i reapplied thick micro while the previous coat was still tacky - i'll check for adhesion this week, but i am hoping that this will lead to just one or two fill and sand routines, just like the post from ugolini says

[Dust]

well the good news is the multiple layers stuck to each other perfectly AND they was no telling the line of demarkation between the two sets of batches

the bad news is - the layer was not thick and uniform enough - will have to fill and sand area again.

next try - 1/8 x 1/16 Stainless steel and 12" drywall knife to try and get one complete uniform layer between the steel and work my way across

Any other ideas?

[Wayne Hicks]

Have you read my Chapter 25 pages? Lots of tips and tricks there, all stolen from others.

12 inch blade is too wide (in my opinion). Very hard to squeeze that much micro. Try a 6-8 inch drywall trowel. Make sure it has a spring steel blade.

Heat the trowel slightly. It makes the micro temporarily less viscous and easier to spread from under the trowel.

Make sure to do a better job during the pre-fill, big fill.

[Dust]

it seems as though that method only catches the localized high spots.

[Nathan Gifford]

Originally Posted by Wayne Hicks

...Heat the trowel slightly. It makes the micro temporarily less viscous and easier to spread from under the trowel...

I wonder if a decapping knife might work (use to decap bee honeycomb before putting them in the centrifuge). I need to check to see if the temp can be turned down.

[Wayne Hicks]

Dust:

You're fighting a losing battle trying to spread the micro perfectly level.

Once cured, leveling out the micro goes amazingly fast with a decent fairing board and decent 36 grit. You'll waste far more time trying to spread the micro perfectly level than you will sanding it to level. It's nearly impossible to level out the dry micro while it's being applied. The best I've been able to do is just to get it evenly spread. You must accept some amount of high spots and ridges. Else, you're going to be at it allllll night!

Trust me, the sanding with 36 grit goes fairly quick, all things considered. Adhere to the "Prime Directive". Just pile it on without it skipping, tearing, or forming air bubble. Apply it all over to be higher than the highest high spot.

Don't go for perfection with the trowel; go for perfection with the board!

[Dust]

Originally Posted by Wayne Hicks

Dust:
You're fighting a losing battle trying to spread the micro perfectly level.

Once cured, leveling out the micro goes amazingly fast with a decent fairing board and decent 36 grit.
Don't go for perfection with the trowel; go for perfection with the board!

i love losing battles - heh heh heh - I'll fight it a little more and see what i come up with. One application and one sanding is my goal, but if i can get it down to 2 i'll be happy. i'm not looking for perfection in appearance of the micro after applying, just enough build. i do have two of these planes to do

[Wayne Hicks]

You'll be mucho happier with the results if you faithfully follow the prefilling and the big filling that I describe on my website.

I didn't do the prefilling on the wings or the first winglet. I thought I was good enough, skillfull enough, patient enough to get enough micro on the parts on the very first try. Wrong, very wrong! I had to recoat the low spots 3 times.

So I followed the process to the letter on the strakes and the other winglet. I prefilled the strakes and I prefilled the other winglet. I did the big fill ONCE and only ONCE. I had only one small, low area to fill where I misjudged the slope of the transition to the wing root.

I was much happier. Took alot less time. I am SOLD on that process.

Try it, you'll love it.