Approved Epoxy for wood repairs ?

blimpy

blimpy

New member
AC 43 mentions epoxy along with Rescorcinol-Formaldehyde Resin in it's discussion of
approved adhesives.

BUT, it says all adhesives used in wood repairs must meet Mil Spec, AMS, or TSO standards.

So, the question is this.

>>> :?: Can anybody name an epoxy that meets one of these acceptable standards, suitable for making repairs
in aircraft plywood ? ?? <<< :?:

I haven't checked the TDS for the West System stuff I have.

I can surely buy the R-F Resin... but the epoxys can also be used as fillers, and the R-F Resin is a thin film
adhesive way I read it.
 
Larry,
The West stuff is good, particularly for making up fillers and casual repairs. But, Rescorcinol-Formaldehyde Resin is what has been holding your Bellanca together for all these years, so you can mark me down in the "Rescorcinol-Formaldehyde Resin has no substitutes" column for critical applications.
Rescorcinol-Formaldehyde Resin is now no longer generally available. It used to be common and sold in hardware stores as Weldwood waterproof glue - (not the Weldwood plastic resin crap). But I think one of the places you can still get R-F is at Spruce.
Ron
 
Yes !

I bought Rescorcinol-Formaldehyde Resin glue from A/C spruce, along with the approved
A/C plywood. in this case 3/32" 90 degree mahogany.

I believe RF Resin is a Thin Film adhesive.

There are places where a repair demands a filler before recovering.

In this case, where I am doing a Plug Repair per AC-43 with the RF, and then need to fill
and sand, as is spelled out there. Clearly demands something different than RF resin.

I've asked Dan T to supervise on this.. so we will see what he says.

----

on the theoretical side.. if you are doing a splayed patch ( beveled plug) unless you get the fit
perfect, an epoxy which can bridge slight gaps, would SEEM to be a better choice.

hence my Question... what epoxy ( if any ) has approval ju-ju ?

(Yeah I like the west system stuff.. well proven )
----

and so the saga continues... :roll:
 
I know for a fact that Shaun Tucker's fuselage is held together with T-88. I do not know of an "approved" epoxy resin. You are correct in that Resorcinol Formaldehyde (RF-typically used in lam beams) resin does not have "gap filling" properties.
The company I retired from made RF, phenol formaldehyde (plywood) and urea formaldehye (particle board) resins. Also some others but the only one that was actually approved for aircraft was Aerolight I think which was made in England. It was a urea formaldehye resin and we were all amazed that it was approved for aircraft. However it was used in the DH Mosquito during WWII. It is the cheapest resin of those mentioned, and I guess they didn't figure the planes would last long enough to debond. Some actually did in SE Asia...warm humid climate plus the stresses put on the planes.
I talked to the FAA a year ago about this,and they told me of weakening of T88 with elevated tempertures, but when I look at what Shaun Tucker does to his plane, and the failures that do occur are in the wood rather than the glue bond tells me that it works pretty well. If failures do occur they are suppose to occur in the wood rather than the bond. So,long story short...it's what they mechanic is willing to sign off and stand behind. The safest from a problem for him would be RF.
 
All,
Years ago, I had a long talk with the chief chemist at "System Three," the manufacturers of T88.
There is no FAA/PMA for this glue or any other epoxy adhesive.....Nor does there need to be. Mr. Kern,
the chemist, told me that T88 meets certain ASTM and SAE requirements for adhesives, as does the products of most major USA manufactured epoxy adhesive manufacturers. This allows the use of this
glue in aircraft structures according to manual 43.
When I had this conversation with Mr. Kern, years ago, he stated that at least 10,000 wings had been
built with T88.....and there was NOT ONE documented glue failure.
The FAA, at the time, did not understand the meaning of the "heat/deflection" test required by SAE testing
procedures and came away with the observation that epoxy glue weakened at 120 degrees F. All epoxies will weaken with enough heat. The point where the glue could give way is up to the manufacturer. Usually this point is somewhere between 250-300F. If your wing is this hot, the airplane is probably on fire and you'd better not be in it.
Dan
 
Dan and Larry, thank you both for your highly educated comments ! That's what I love about this group.

A/C spruce sent me something called Casco Phen or some such.
No Mixing Instructions came with it !!

Yup. AC43 mentions several ways that adhesives can be tested and approved.

Since I only need it for a filler, and very little of that, I'm not too concerned about it anyway.

:)
 
I have been looking into wood resin as well. Was this the resin you received from Spruce? https://www.aircraftspruce.com/catalog/pdf/cascophentech.pdf
Did it work okay for your needs? Just curious before I order some for me.
Thanks for your input,
JP
 
CascoPhen.... only FAA approved Phenolic Rescorcinol type glue they list.

However it has been pointed out that T-88 and other West System epoxies are probably
also approved for some repairs.

It think what you use depends on What you are doing.. and if your are working in concert with an
A&I , you want to consult with them and use what they want.

For example, there are spots where wood end grain is exposed ( look at the are where your flaps and ailerons
come together). This is an area where you can choose to use a varnish.. an epoxy, or a paint. Or some of both.

When it comes to wood work on the wing.. the Casophen is what was used originally, so it looks right,
and is right... although it is a considerable pain in the ass to use for a patch compared to epoxy.

I've had a big learning curve just to lay out and make a wing skin doubler and plug per AC-43.
Getting the right tool ( scroll saw) made all the difference... so first items tossed which represented hours of work,
in favor of oh-so much better copies made with the right tool, in minutes.

In the same vein, figuring out how to ( and how NOT to) actually do the repair has eaten all sorts of time.

One only goes through this is one is too poor to hire an expert to knock it out in a few hours.. instead of the months
it is taking me !

If you don't have a heated hanger/shop maintaining a MINIMUM of 70 Degrees F for 12 hours or more... you MUST find a way to heat the work, the parts, and the rescorcinol . For me that mean using a heating pad, generator or batteries, and speeding up the whole process in order to achieve a safe bond.

I will be doing some test bonding in the workshop at home, before I try this on my airplane !

So, I will let you know.

What I know about this stuff now is that it only wants a hairline thickness of adhesive applied to clean smooth very tight joints, it requires pressure until cured at specific temperatures. It does NOT fill voids, or mistakes like epoxy, or bound to rough surfaces.

Hence the comments on one needing to be a "violin maker". If you were trying to make a splayed patch.. I would say that is pretty much true. A plug patch.. not so much.

Read AC-43, it contains plenty of info on using rescorcinol... You will get NO instructions from AC Spruce !

Plenty of guys on here have experience using this stuff. Clearly I am a rank beginner trying to learn how to do
safe and acceptable work. :roll:
 
blimpy said:
It does NOT fill voids, or mistakes like epoxy, or bound to rough surfaces.
Click to expand...
Gawd, I hope not. If someone is doing work that results in voids and mistakes.. best to keep that plane on the ground and tattoo those words on the knucklehead's forehead.
Approved or not, epoxy is never a solution nor an excuse for sloppy workmanship.
 
Ditto on wood repair gaposis. In the best of worlds, the glue line will be almost invisible if you shave across it . But, this requires good access to the repair area and good pressure on the glue line during cure. One can only do what is possible, and luckily, many skin repairs are not too critical.
I would say, though, that skin repairs on or near the spars are in that critical category and every effort to retain strength and continuity must be made. The skin in the spar area is actually loaded higher than the spar itself, and any failure would tend to occur at the skin. The rest of the wing skin, though, takes relatively low torsional/shear loads and in some areas is just along for the ride holding the paint up.

Larry, you didn't say where your wing skin repair was or how big, both things that matter. You did say that you would use a plug patch. I wonder how good resorcinol is gluing the doubler to the treated/varnished under surface? Resorcinol really wets wood which makes it very reliable but epoxy might just be the best choice for bonding to other than raw wood for this type of repair. Methinks the FAA should have a TSO for an epoxy wood glue product available. Same with fabric. Should be a TSO for polyester like there was for cotton.
I should mention that I did my first spar splice when the FAA was the CAA , AC43-13 was Manual 18 and casein glue was plenty good. So, while I am not an authority, the subject is important for us wood wing airplane owners, and for me, a blast from the past.
Larry, stay warm.
r
 
doc... using the Plug method for skin repair, after you glue and screw in the doubler backing , you surface glue the plug to the doubler, then to paraphrase AC-43.." use suitable fillers". One wants to make the nicest fit of the plug, but it isn't a splayed patch that relies on precise fit to the skin as the source of bonding strength.

So, in that context , an epoxy would be the appropriate filler to eliminate any gaps between the plug and skin, and to give perfect smoothness, before you patch the fabric over it.

At least that is my current understanding of the plug patch as detailed in AC-43.

every day I discover I was wrong .. so there you go.
 
2 1/4 inch "round" hole after cleaning up with a hole saw and a sharp blade.. top of wing.. sufficiently distant from ribs and spars to allowa plug patch with doubler per ac-43. No varnish inside wing skin... factory finish 1947. I will use sandpaper inside to lightly clean the underside of the skin.. mostly because of concerns about any accumulated dirt that could interfere with bonding.

Doubler is a disc with a hole in the center ( proportioned per AC-43) and a cut into center so it can "cork screw" through the existing hole ( trick in AC-43 shown in diagram but not described).

After prepping the skin.. and pre-drilling the skin using a template, the doubler and the skin get a light coat of glue,
the doubler gets held in place temporarily while it is screwed to the skin with nine small screws spaced 1 inch apart.

Everything above pre-heated. Whole mess heated with the heating pad... to 100 degrees F... minimum of 4 hours.. or whatever is called out.

Next day.. remove the screws. Glue plug to doubler..weight and same heat regime.

Next day... fill any gaps between edge of plug and skin it is plugging :oops:

Next day.. apply pre-prepped fabric patch.

That is the Plan.. the application of the glue in the correct amount and way.. I need advice on.. or luck.

:roll: :oops: :mrgreen: :?: :roll:
 
You must log in or register to reply here.
Back
Top