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-   -   Seal Balsa before applying epoxy ? (http://www.rcuniverse.com/forum/composites-fabrication-repair-97/585628-seal-balsa-before-applying-epoxy.html)

sportflyer-RCU 02-26-2003 06:25 PM

Seal Balsa before applying epoxy ?
What is the main purpose of sealing the balsa skins before applying the laminating epoxy ? Is there a big advantage of doing so? The suggested sealants in this forum are 1) 50% Thinned Nitrate Dope, 2) Hair spray & 3) BalsaRite . Can I also use Shellac, Clear Laquer or any other wood sealants in cans or spray found in Walmart for sealing? Thanks

Ben Diss 02-26-2003 07:11 PM

Seal Balsa before applying epoxy ?
The purpose is to prevent the adhesive from soaking into the wood. The goal is to save weight by using less epoxy.

The real question is: Do you want the epoxy to soak into the balsa (strenght vs. weight) and is the material being used to seal the balsa adding as much weight as would be saved in epoxy?


probligo 02-26-2003 09:34 PM

Seal Balsa before applying epoxy ?
No argument with Ben Diss except...

I have been making a practice of applying 1 coat of 50/50 weak dope and then scuffing the surface fuzz off with #400 w&d. The resulting surface is still "rough" enough for the epoxy to get a first class "key" without the penalty of soakage into the wood.

Savings? Very difficult to determine. But it makes me feel good.

turbines 02-27-2003 03:50 AM

Seal Balsa before applying epoxy ?
Probligo has it right! This works very well.

Ben Diss 02-27-2003 06:04 PM

Seal Balsa before applying epoxy ?
I'd really like to know which way is better. I know that we all have opinions, but has anyone done the math to see what effect on the strength and weight this process has?


sportflyer-RCU 02-27-2003 08:06 PM

Seal Balsa before applying epoxy ?
Is there a difference between using Nitrate and Butyrate clear dope? I have easy access to the latter. Alternatively if I use Balsarite do I need to thin it , if so what kind of thinner to use ? Laquer? Tks

probligo 02-28-2003 12:11 AM

Seal Balsa before applying epoxy ?

It is always nice to be scientific about this. See my earlier post - "I don't know if it has much effect but it makes me feel good."\

Even if there were a scientific/mathematical study done there would be too many variables to make it worthwhile.

Comes to mind -
  • balsa density/porosity
    epoxy used
    epoxy temperature
    vacc'd or not
    and many more I can not think of at present.

probligo 02-28-2003 12:20 AM

Seal Balsa before applying epoxy ?
I need enlightenment (I have never had the difference explained) on the difference between Nitrate and butyrate dope.

I work with "weak dope" which is "strong dope" thinned out somewhat. I also have "non-shrink dope". Oh, "strong dope" is like well thinned out balsa cement. I use amyl acetate as a plasticiser (damned dangerous that stuff is too...) in very small quantities.

For this purpose I always used thinned 50/50 weak dope. Thinners? Acetone/toluene mix. No plasticisers, no fancy stuff, I want that sealer to get well into the balsa surface.

Jimbonk 03-08-2003 12:20 PM

Seal Balsa before applying epoxy ?
Since I started using the following method for applying to Wood wing skins I've found that there is no real need to seal the wood. I use a 3" plastic trim Paint roller to apply the Epoxy to the Skins. You can buy these in Most Home Centers . These little rollers have a Plastic Frame and come with a small plastic paint tray and cost about $3.00 . Replacement rollers are about $1.50/ pair. ( just make sure that you remove the roller from the frame before you leave the shop)
After mixing the epoxy I add some Phenolic Micro Balloons . This gives the resin some "color" and the Micro Balloons tend to get "Stuck" in the pores of the wood. After mixing the Micro Balloons , pour the resin into the tray and wetout the roller. Now just "Paint" the resin onto the skins. It takes about 60 seconds to do both top and bottom skins on 38" span wing panel . About 90 seconds for a 60" one. The roller makes it easy to get epoxy right to the edge of the skins. Have a pc. of cardboard handy and you will be able to "dry out" the roller on the cardboard and "suck-up" any excess resin from the skins.

Before you leave the shop pour out as much resin as you can from the tray . Let the rest just cure in the tray, and you will be able to reuse the tray over and over.

Jim Bonk

Ben Diss 03-12-2003 01:49 PM

Seal Balsa before applying epoxy ?
I did a little experiment last night. I took four pieces of 3/8" x 6" x 12" balsa and laminated 3/4 oz glass to each piece. I used four different techniques.

1. applied peel ply over glass, then mylar
2. applied peel ply plus 2 pieces of paper towel, then mylar
3. applied mylar over glass
4. applied one coat of sanding sealer to wood, then mylar over glass.

I weighed the test pieces after I cut them. For piece #4, I let it dry one hour and weighed before and after. For all test pieces, I applied a measured 1/2 oz of West 2hr. epoxy directly to the wood, laid the glass on top and then matted the excess epoxy with a paper towel. I applied 20" of vacuum and left the parts in the bag for 12 hours.

Before weighing them after curing, I trimmed all excess glass to the edge of the balsa. Here is the additonal weight (above the bare balsa) for each part:

1. 1.7g
2. 1.7g
3. 2.1g
4. 2.1g (0.3g added by sealer)


1) Paper towel in addition to peel ply had no effect on weight in this test. I'm going to weigh the peel ply and paper towel+peel ply to see if more epoxy was actually removed. It could be that absorbing some excess epoxy prior to the application of peel ply negated any additional weight savings expected by the addition of the paper towel.

2) Sealing the balsa did not result in any weight savings due to the additional weight of the sealer. This test needs to be repeated with different sealing materials and different application methods.


- What sealing material should I test and how should it be applied?


zotandy 03-13-2003 11:51 AM

Seal Balsa before applying epoxy ?
Hi Sportflyer

I've used Shellac for several years in this type of application. It is not so much a problem of weight but if your thin layer of epoxy disappears into the soft balsa you have a dry joint. I've seen a total de-lamination with drastic results when this has not been done. Tarmac is not soft. If you are worried about strength some sealing is possible by mixing a small quantity of Microfibers into the resin, West System Number 403, but you are still not sure how good the joint is?? I thin the full strength shellac about 30% with Methanol. As it is alcohol based it dries in about 30 minutes in a warm room, I spread it with a bit of cloth, quicker than a brush but you end up with furniture colored fingers for a day or so.


KenChoo 03-18-2003 12:31 AM

Seal Balsa before applying epoxy ?
I think there's a point that's missed being said explicitly here, although it was behind a comment already made. It's not just about weight savings, it's about being able to ensure a good enough bond between the wood skin and the foam core while using less epoxy. Without sealing the skin first most of the epoxy would tend to soak into the wood versus the foam so the bond isn't as good with the same amount of epoxy - meaning you'd have to use more epoxy, hence adding weight. By sealing the wood, you limit a bit how much the epoxy soaks into the wood (of course it needs to do that to establish a bond), so the epoxy can grab into the foam as well, and you can be surer of a good bond between foam and wood with less epoxy!

Hope this wasn't too confusing. Plus I may be wrong - anyone have another view about this?


Jimbonk 03-18-2003 12:30 PM

Seal Balsa before applying epoxy ?
Most builders use the " Pour on a Stripe of Epoxy, and use a Squeegie to spread it around" method. Using a Squeegie is going to have the effect of forcing the epoxy into the wood, so in this case maybe using some kind of "sealer" is a good idea. If you use the "Roller Method" , then I think that Sealing is probably not necessary. If you insist upon sealing the wood , then maybe you should try Spraying a very light coat of the sealer on. Very Light meaning just a "Mist" coat.

Jim B

TOYMAKER 03-18-2003 04:17 PM

I dont know why you guys are wasting your time with epoxy to sheet wings. Use Pro-Bond, Urethane glue to do this. It is sooooo much easier then epoxy and penetrates by foaming action triggered by a light aplication of moisture prior to sheeting. No mixing either just a one part glue. This glue penetrates better then any epoxy I have ever used. I dont vacum bag so this method would probably be prefered if you cant bag them.

In two different sets of idenical wings that I have done I have come to the conclusion the sealing can save weight.

On one set I used 50/50 thinned Nitrate dope and sanded lightly after this dried. I have even used cheap hair spray to seal also. Apply just enough Pro-Bond to make a shiny sheen spreading it evenly with a credit card(one thats maxed out). I have seen a savings of an ounce per wing panel by this method. These are 475 sq in per panel. I am convinced that it works and it has not impared the strength or sealing ability of the glue.

Ben Diss 03-18-2003 04:28 PM

Seal Balsa before applying epoxy ?
Toymaker- We're talking about applying fiberglass on top of balsa. Probond isn't good for that.


TOYMAKER 03-18-2003 04:35 PM

I guess I should have put my glasses on.....
I was refering to a most recent post by KenChoo.
Carry on fella's

But it works good for sheeting them... heh! heh!

probligo 03-18-2003 06:26 PM

Seal Balsa before applying epoxy ?

It is not a question of "soaking in" as "having a key to hold".

Go back to my original post, I use dope and then just a quick scuff with sandpaper to remove any "loose" stuff on the surface. At this point the sealed surface is still quite "rough" to the touch. It is that roughness that keys the epoxy to the surface. It is also the reason why one must use a low viscosity resin.

Personally I suspect that the majority of resin bond failures are the result of using the wrong resin, or of incorrect mix, or forced first set.

Ben Diss 03-18-2003 09:27 PM

Seal Balsa before applying epoxy ?
BTW, how much vacuum is everyone applying? I used 20" in my experiment. I suspect that less pressure might result in less penetration into the wood by the epoxy. Not sure if that's good or bad.


probligo 03-18-2003 11:35 PM

Seal Balsa before applying epoxy ?

I am a pour and wipe practitioner.

CafeenMan 03-19-2003 06:06 AM

Seal Balsa before applying epoxy ?
I don't get it. I've never sealed wood before applying fiberglass and haven't had any problems. Are you guys having problems with your cloth peeling off in flight or something? :confused:

zotandy 03-19-2003 10:54 AM

Seal Balsa before applying epoxy ?
Info for Ben

I use only about 7" Hg. Much more than this on a heavily honeycomb wing core with 12 oz. White foam can result in crushing the core. Been there, done that, got the tee-shirt.


probligo 03-21-2003 12:44 AM

Seal Balsa before applying epoxy ?
Well, speaking personally Cafeenman, if I can save 5 grams for a 10% strength gain on a 35 gram airframe it is heaven sent.

If I can save 10 grams on a 80 gram airframe...

The need for the glass is 'cos I don't throw too good and I frequently bust wings :D:D that is why I need them stronger :D:D

The current project is glass/cf/depron foam - a challenge that is.

mikenlapaz 05-18-2003 05:06 PM

Seal Balsa before applying epoxy ?
1) your sealer creates a chemical bond with the balsa;
2) you ask the epoxy to chemically and mechanically bond to the sealer (and any scuffed areas to bare balsa;
3) is not the bond only as strong as the sealer/balsa connection?

would you epoxy over a interior surface joint that had paint on the surfaces and expect the the same strength as to bare wood?

probligo 05-18-2003 05:39 PM

Seal Balsa before applying epoxy ?

You are right for a perfectly smooth surface at a microscopic level.

However that is also ( in my case at least) that is an unachievable ideal.

There is a physical bond as well as the chemical and it is this that the discussion centres on. That bond arises from the epoxy "soaking into" the suface of the substrate. It does help the bond. The question is kinda like "when is this too much of a good thing".

So, if you get a piece of 1/4" soft balsa, and put a coat of laminating epoxy on it, the cut it in half after the epoxy has set you could be able to see how far the epoxy has penetrated. Again the question is "how useful is this?"

Darrinc 05-19-2003 12:54 AM

Seal Balsa before applying epoxy ?
The proper method of making a light A-Sandwich (skins with a core) are as follows:

1. Lay the skins up first, you need to pull as much vacumm as you can to consolodate the lay-up. Aim for a 38% resin to weight factor and do not let the lay up bleed, completely seal the lay-up with a top caul plate and teflon tape (or clear tape) around the edges, then poke a hole with a pin in the tape every 6 inches. (to burp the lay-up when vacuum is applied) Then envelope bag your lay-up and pull 26.4 ~27.2 inches of vacuum.

2. Bake your balsa for 3 hrs at 200F to drive off any moisture.
(moisture is the # 1 reason for laminate failures)

3. Once the skins are fully cured, skuff up the bonding side of the skin with a sand blaster or 100 to 180 grit sand paper. REMOVE ALL GLOSS!!!!!

4. Mix up a thick slurry of micro balloons and epoxy. Spread a thin layer (15 thousands) of this slurry on to the bonding side of the skins, then install your core material and pull approperate vacuum to not crush the core.

This method will produce a low weight, high strength, mil spec part every time.


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