Was wondering if finishing resin could be used instead of regular epoxy. Anybody ever done it ?

Allan Perret

hi

i use two pack expoxy resin from WEST systems and they are light and very strong

dont use contact adiesive as it never goes hard and will buckle the skins if it get hot under the normal sun
paul

3M 77 in the black can is my choice. The new formula in the red can is very tricky to use but can be used. I've never had a skin come loose with it. To use the red can just hold it 2 feet from the core and dust on a couple coats blowing air on it as it dries. This will get rid of the harmful solvents before melting the core.heavier coats can be put on when the core is covered as the dry glue protects the foam. Put a couple heavy coats on the skins on a 90 degree bias/pattern and let dry for the required time. Roll the cores onto the skins and put the assembly into the shucks, aligned carefully, and weight it down and let cure overnight at least. I've never had good luck using any kind of epoxy. Dave Brown Sorgum and Sig contact cement works good too.

Perry Rose

>Was wondering if finishing resin could be used instead of
>regular epoxy. Anybody ever done it ?
>
>Allan Perret


Ron B.
F4Fguy

Allan,

I'm not quite sure of your meaning. Are you looking for an adhesive to apply the skins to the core, or to finish the skins?

If the former, the type of resin is the determinate factor. Any epoxy based resin will work, some better than others. Polyester based resins, such as K&B, etc., will definitely not be suitable. The polyesters have solvents in them which will disolve the styrene foams we use for cores. Sure, it may be possible to seal the surface of the core to avoid penetration of the polyester, but why bother? it's another step, it's additional weight, and it's "iffy" at best.

Epoxy, on the other hand, is safe, light (if applied properly), easy, and reliable. West Systems, as noted by Paul above, have a number of suitable epoxies. I find the slower ones easier due to thier long "open' time which allows both time to remove excess resin and time to adjust position if needed.

Ron B.

Edit to add PS

PS: As I was writing, Perry wrote reccomending 3M 77. What he says is mostly true. I have two major objections to this product:

(1) It will get brittle with age, and eventually fail at the bond line.

(2) It's much heavier than properly applied epoxy.

R.B.

Epoxy is the best method bar none and epoxy finishing resin works fine.
Make sure the cores are properly sanded, dusted and vacuumed to remove all dust and that the skins are baby bottom smooth, both sides will save lots of work later.
Never ever put the epoxy on the foam, you are just adding unnecessary weight.
You will be amazed at just how little glue you actually need. Pour 1/2 to 3/4 ounce on the skin and using a scraper spread it all over the surface. Rather than drag it, push it.
Then try and get it all off!!! yep, that's how little you need. I leave the skins wet along leading and trailing edges and over spars if used, i.e. shiny rather than matt.

I don't know how many Pylon ships I built back in the 80's and 90's using this method.

STAY WELL AWAY from contact and latex glues (use contact for making decent sanding blocks!). To many problems and way way way too heavy and prone to lifting, more difficult to repair too

After the balsa skins are prepared, put a couple of coats of nitrate dope on the surface you are going to apply the glue to: this prevents the balsa from soaking up more epoxy (what one uses to attach the balsa skin to the foam) than is really necessary.This works nicely, some spray hair spray on the foam, this is supposed to serve the same purpose as the nitrate dope, anyone tried this? Stir that epoxy well!

>After the balsa skins are prepared, put a couple of coats of
>nitrate dope on the surface you are going to apply the glue
>to: this prevents the balsa from soaking up more epoxy (what
>one uses to attach the balsa skin to the foam) than is really
>necessary.This works nicely, some spray hair spray on the
>foam, this is supposed to serve the same purpose as the
>nitrate dope, anyone tried this? Stir that epoxy well! :sp:
>


Ron B.
F4Fguy

You don't need the dope. It's at least as heavy as the "soaked in" epoxy if the epoxy is applied and squeegeed properly. Same goes for the hairspray. One of the reasons epoxy works so well is that it penetrates, especially if you use a very slow epoxy and if the assembly is bagged. This "soaking in",if you will, gives a broader, more graduated bond line with less tendency to fatigue the foam at the bond line.

It's quite easy to control the epoxy weight. simply weigh out the amount you need and apply only that quantity. It may seem inadequate at first glance, but just spread it around wiith the squeegee. You'll be amazed how much you'll squeegee off as excess. All you want to leave on is enough to coat the surface of the skin. If you can see any wet spots you've left too much on. Squeegee it off.

Incidentally, I highly recommend vacuum bagging there's simply no way to pile enough books, magazines, cinder-blocks or bricks to give the total pressure or the even pressure of even a relatively low vacuum. think about it, 10"hg = Approx. 4.9PSI x two sides of a 600" wing =
circa 5000#. Not that you need that much, 5-7"hg is plenty. The first wing I ever bagged was done with a Shop-Vac and garbage bags.

Ron B.

hi all
what ever you do if you use expoxy or contact becareful the fumes that some foams give when in contact with some glues you only know about when you wake up stuck to the cores
paul

>>After the balsa skins are prepared, put a couple of coats
>of
>>nitrate dope on the surface you are going to apply the
>glue
>>to: this prevents the balsa from soaking up more epoxy
>(what
>>one uses to attach the balsa skin to the foam) than is
>really
>>necessary.This works nicely, some spray hair spray on the
>>foam, this is supposed to serve the same purpose as the
>>nitrate dope, anyone tried this? Stir that epoxy well!
>
>>
>
>
>Ron B.
>F4Fguy
>
>You don't need the dope. It's at least as heavy as the
>"soaked in" epoxy if the epoxy is applied and
>squeegeed properly. Same goes for the hairspray. One of the
>reasons epoxy works so well is that it penetrates, especially
>if you use a very slow epoxy and if the assembly is bagged.
>This "soaking in",if you will, gives a broader, more
>graduated bond line with less tendency to fatigue the foam at
>the bond line.
>
>It's quite easy to control the epoxy weight. simply weigh out
>the amount you need and apply only that quantity. It may seem
>inadequate at first glance, but just spread it around wiith
>the squeegee. You'll be amazed how much you'll squeegee off as
>excess. All you want to leave on is enough to coat the surface
>of the skin. If you can see any wet spots you've left too much
>on. Squeegee it off.
>
>Incidentally, I highly recommend vacuum bagging there's simply
>no way to pile enough books, magazines, cinder-blocks or
>bricks to give the total pressure or the even pressure of even
>a relatively low vacuum. think about it, 10"hg = Approx.
>4.9PSI x two sides of a 600" wing =
>circa 5000#. Not that you need that much, 5-7"hg is
>plenty. The first wing I ever bagged was done with a Shop-Vac
>and garbage bags.
>
>Ron B.
I was in a hurry once and it was too cold to brush on the dope, so I applied the epoxy without first applying the nitrate dope like Randy Smith reccomended, and the skins did not stick very well, highly reccomend using nitrate dope first.If one uses real light, almost punk wood, it tends too soak up alot of the epoxy. We have vacumm bagged wings, it works real well, takes a bigger investment of time and money.Rather messy, but that was probably our fault!

>I was in a hurry once and it was too cold to brush on the
>dope, so I applied the epoxy without first applying the
>nitrate dope like Randy Smith reccomended, and the skins did
>not stick very well, highly reccomend using nitrate dope
>first.If one uses real light, almost punk wood, it tends too
>soak up alot of the epoxy. We have vacumm bagged wings, it
>works real well, takes a bigger investment of time and
>money.Rather messy, but that was probably our fault!


Ron B.
F4Fguy

A couple of points:

If it's too cold for nitrate dope, it's waaayyyy too cold for epoxy.

The idea that significant soaking occurs, even with "punk" wood is a myth. What little does occur is benificial.

Why would anyone use "almost punk" wood as a skin? All of the strength is in the skin and all the stiffness is in keeping the skin from deflecting (buckling) by bonding to the foam core. Medium to hard 6#-8#, 1/20" or 1/32" is as light as 4#-6# 1/16" or 3/32", and is much superior structurally.

I stand by everything I said in my previous post.

Ron B.

>>I was in a hurry once and it was too cold to brush on
>the
>>dope, so I applied the epoxy without first applying the
>>nitrate dope like Randy Smith reccomended, and the skins
>did
>>not stick very well, highly reccomend using nitrate dope
>>first.If one uses real light, almost punk wood, it tends
>too
>>soak up alot of the epoxy. We have vacumm bagged wings,
>it
>>works real well, takes a bigger investment of time and
>>money.Rather messy, but that was probably our fault!
>
>
>Ron B.
>F4Fguy
>
>A couple of points:
>
>If it's too cold for nitrate dope, it's waaayyyy
>too cold for epoxy.
>
>The idea that significant soaking occurs, even with
>"punk" wood is a myth. What little does occur is
>benificial.
>
>Why would anyone use "almost punk" wood as a skin?
>All of the strength is in the skin and all the stiffness is in
>keeping the skin from deflecting (buckling) by bonding to the
>foam core. Medium to hard 6#-8#, 1/20" or
>1/32" is as light as 4#-6# 1/16" or 3/32",
>and is much superior structurally.
>
>I stand by everything I said in my previous post.
>
>Ron B.
>I have to dope things outside on a balcony, it's about 15 degrees out there. To each his own, I prefer using nitrate on the skins, as does about everyone else I know. It is superior to not using. Punk wood (almost) is all right if treated properly, in my opinion.
>
>
>
>
>
>
>
>

>>I stand by everything I said in my previous post.
>>
>>Ron B.


>>I have to dope things outside on a balcony, it's about 15
>degrees out there. To each his own, I prefer using nitrate on
>the skins, as does about everyone else I know. It is superior
>to not using. Punk wood (almost) is all right if treated
>properly, in my opinion.


Ron B.
F4Fguy

As you say, that's your opinion. The facts, however indicate otherwise. There is penetration, to some extent, in both cases. In fact, the lower viscosity nitrate will penetrate deeper. The weight of the solids in the two materials is almost the same. Epoxy has much better properties than cellulose nitrate, and nitrate, even non-tautening, continues to shrink over time leading to stresses in the bond line.

For the same total weight of wood, denser, thinner skins are stiffer and stronger than thicker, low density "punk" wood.

Ron B.

Well, I have been doing foam wings for about 15 years, and the adhesion was better with a coat or two of nitrate dope on the skins prior to applying the epoxy. Hobby-poxy used to make a 3 hour epoxy, I thought it was the best for sheeting wings. As far as using almost punk wood goes, when one is out of work and places an order to Balsa USA for some light wood 6" in width and they send some really light wood, one has to make do with what is available.

Hey Ron:
Would like to give the vacuum bagging a try.
I already have a vacuum pump, one of the AC evacuation types.
What is a good source for the bagging supplies ?

Allan Perret

>Hey Ron:
>Would like to give the vacuum bagging a try.
>I already have a vacuum pump, one of the AC evacuation types.
>What is a good source for the bagging supplies ?
>
>Allan Perret


Ron B.
F4Fguy

Allan,

Try Aerospace Composite Products. George Spahr is the founder and is practically the father of composites in aeromodeling.

Ph. (925)443-9200


Ron B.

Nobody here has mentioned using polyurethane glue in vacuum-bagging. I've done a bunch of sailplane wings with epoxy, which works very nicely. I've also done some wings with polyurethane, and I can't see a weight difference, and there's no mixing with the polyurethane, just squeegee it on and pop the whole thing into the vacuum bag. Much less preparation hassle.

Now, I HAVE NOT DONE THIS on model airplane wings, but I've done it, under guidance from a luthier of a zillion years' experience, and I've glued up guitar tops/backs with their respective braces using Titebond and popped THEM into the vacuum bag, and the glue set within a couple of hours. Removing the air via continuous vacuum also lowers the boiling point of the water in the glue, so I was told, and the vacuum cooks off the water most promptly. Worked when I did it. Do need to leave the vacuum pump sucking away.

Mention above about using an automotive air-conditioner-servicing vacuum pump. A vacuum pump's duty cycle needs to be continuous for many hours, like twelve....If the pump isn't up to that, it's time to talk Aerospace Composites.

Like all other operations during construction, experience and practice go a long way towards developing proficiency.

Peter Havriluk

>Nobody here has mentioned using polyurethane glue in
>vacuum-bagging. I've done a bunch of sailplane wings with
>epoxy, which works very nicely. I've also done some wings
>with polyurethane, and I can't see a weight difference, and
>there's no mixing with the polyurethane, just squeegee it on
>and pop the whole thing into the vacuum bag. Much less
>preparation hassle.
>
>Now, I HAVE NOT DONE THIS on model airplane wings, but I've
>done it, under guidance from a luthier of a zillion years'
>experience, and I've glued up guitar tops/backs with their
>respective braces using Titebond and popped THEM into the
>vacuum bag, and the glue set within a couple of hours.
>Removing the air via continuous vacuum also lowers the boiling
>point of the water in the glue, so I was told, and the vacuum
>cooks off the water most promptly. Worked when I did it. Do
>need to leave the vacuum pump sucking away.
>
>Mention above about using an automotive
>air-conditioner-servicing vacuum pump. A vacuum pump's duty
>cycle needs to be continuous for many hours, like twelve....If
>the pump isn't up to that, it's time to talk Aerospace
>Composites.
>
>Like all other operations during construction, experience and
>practice go a long way towards developing proficiency.
>
>Peter Havriluk

Ron B.
F4Fguy

Peter,

You're right, urethane works well. The caveat is , it has to be vacuum bagged. If not, it will foam enough to lift the skins.

As to pumps, most automotive AC service pumps are designed for long duty cycles. I've done a great number of wings with mine, and have no trouble pulling 22+" after 30+yrs.

Ron B.

What would be the cure time using polyurethane glue and vacuum bagging. Also, would there be any need to intrduce any moisture ? I've heard there has to be some moisture for it to cure. I would think that both the balsa skins and the foam cores are super dry..

Allan Perret

The lightest misting of water on the foam cores will introduce plenty of moisture. Don't need much, but do need some. Setting time varies with ambient conditions. I've never had a problem with leaving cores in the bag overnight with the pump working.
Peter Havriluk

"You're right, urethane works well. The caveat is , it has to be vacuum bagged. If not, it will foam enough to lift the skins."

It will? I have done several wings that were done the old fashioned way with a stack of books and magazines and didnt have any real problems other than the foaming coming thru the balsa (which is why you MUST use wax paper as a release agent). It does push down into the white foam nicely making it possible to take a bit more out if you core the wing after sheeting it. Just make sure you spread the poly glue as thin as you possibly can. You dont need more than a faint sheen left on the wood before clamping it all down. I cut my own wings and do my own coring all by hand.

Bob Furr

"See ya in the circle!"

One more comment...this on poly glues and water. In the summer the humidity here in Nebraska is high enough to set of the poly glues. In the winter I lightly mist the foam cores and not the wood. The more water the more foaming from my experience. I do leave everything clamped for several days and check the edges before taking all the weight off.
Bob Furr

"See ya in the circle!"

Sig sells 3M "77" spray adhesive. They state that it can be used to bond foam cores to balsa skins.

It appears that they no longer make "Core Bond" which was a water based contact adhesive. I used it once and it worked well. It smelled kind of like pineapple juice (to me, anyway).

But as with all contact adhesives, you better do it right the first time, cuz there aren't any do-overs...!!!

Bob

"I might have had amnesia once...but I can't remember."

Hello Dude

Can recommend the use of preparatory dope on skins before epoxy.

The main reason is, depending on the quality of the balsa or obechi skins, you can get a problem with debonding due to the epoxy being soaked up into the skin. Especially true with obechi which often has grain holes all the way thru to outer surface.

My experience with nitrate dope thinned quite a bit was to prevent any bleed thru and results in a nice bonded skin.

Actually, I have been using this method on thin glider wings and include light weight glass cloth to stiffen them up without having to introduce carbon spars.

Definitely worth the little extra work to make sure the skins don't "bubble" up in places. The dope is compatible with epoxies in general and I have used both SP very slow and Fibre Tech 12hr "lotion" which is very low viscosity.

You can add small amounts (maybe around 50% by volume not weight obviously) of the new supa lite fillers to the epoxy mix and this gives a lighter bond depending on the type of foam. Don't use micro ballons as these are 5 times heavier than filler and also weaken the cured epoxy.

The filler is called "Super Light Fairing Compound" and we get it from Fibretech GB (good stuff !)

Happy sticking. Pete in England.