Strengthening balsa with CA techniques

[maxpower]

What is the best method to spread CA on a bulsa fin? I have tried this with a scrap piece of balsa but if you do not move quickly the scrap gets glued to the fin. After a short bit glue builds up on the scrap and you can't spread the CA smooth anymore. If you get too much CA in any spot and it starts to cure it generates a lot of heat and my brand bubbles also leaving a rough surface. What is a good technique for spreading CA?

Thanks

 

[m85476585]

I would use thin CA (at least that's what I do for nose cones). It will soak in and spread out enough that you shouldn't have to spread it.

I prefer paper lamination because it is cheaper, safer, and does not become brittle over time (which CA does). It also makes finishing easier.

 

[dbarrym]

I do this on the fins of most of my LPR projects. It doesn't really strengthen the balsa that much, but it does make it torsionally much more stiff and toughens up the surface (but it is brittle - doesn't "dent" as much as "ding" or crack after the CA treatment). It also makes a good grain filler; I go straight to primer rather than having to go through several coats of sanding sealer (but I do sand the bare wood down to 400 grit first).

I use the cheapest plastic sandwich Baggies and my index finger, switching to a different spot on the baggie for each application (fin side, etc). I use a CA bottle with the Teflon tube applicator and use the tubing as a kind of "brush" to spread the CA around, and then smooth it with my finger in the baggie. Don't let the CA pool and only apply as much as the the wood will absorb; I also keep some paper towels handy and use it to wipe off any excess.

You don't want any runs or drips as CA is much harder than balsa to sand! I then finish sand with 320 and 400 grit and I'm ready for primer. The fin in the attached pic was done this way...this is after 10 or so flights. Only one small nick (this is an 8 oz, 3xC cluster that has only a 12" chute - comes down quick).

 

[Stymye]

I just use thin cya and no spreader. it soaks in like water.

I don't know if the technique matters but mine is to run the tip of the bottle back and forth over the balsa working out to the edge of the fin .
If theres any dry spots, I go back and fill them in.

 

[gerbs4me]

I have never tried this method, I've just coated the balsa fins with white glue. Next time I'll try this method

 

[Micromeister]

I use both thin and medium CA's for soaking depending on the surface to be soaked. Thin CA needs no applicator just wash on it soaks in like a sponge. Medium CA's on Large surfaces is a bit more of a challange to get in so I fold up a paper towel which is used as a mop to spread the adhesive. Watch your fingers as the medium will soak the towel and generate a good bit of heat if your not careful. Make sure your in a well ventelated area and never keep your face..Nose over the work. The towel mop will sometimes "smoke" but i've never had one actually ignite. Work on a cutting board or better a glass surface so any spills or dirps can be razor bladed off easily.
Hope this helps.

 

[flying_silverad]

I've used all kinds of different styles for spreading CA on fin stock and other balsa components. For me, using the baggy method works best only because it seems to cut down the sanding time a little and you seem to be able to geta more uniform coverage in tight spots.

 

[Tedman]

Originally posted by stymye
I just use thin cya and no spreader. it soaks in like water.

I don't know if the technique matters but mine is to run the tip of the bottle back and forth over the balsa working out to the edge of the fin .
If theres any dry spots, I go back and fill them in.

thats the same method I use Hey look this was my 200th post

 

[Stymye]

it raises the grain so you will have to sand it no matter what

 

[Mad Rocketeer]

I applied thin CA (the water thin Bob Smith stuff) to my Stormcaster's fins and the rear of the body tube a night or two ago, and it really didn't appear to soak in much at all. Not at all what I'm used to. When I treated the front of the body tube and the rear of the motor mount tube a few days earlier, it soaked in right away and behaved as I expected. This time, it acted like water on plastic. It soaked in on just a few isolated spots along the tube edge and didn't seem to absorb in the fins at all. I spread it with a paper towel, though baggies are superior for this, IMHO. I got a thin layer of CA on the fins and most of it soaked into the paper towling. Once the CA cured and I gave it the most cursory sanding (10 or 12 light strokes), I couldn't see any evidence of the CA on the surface, except in the grain depressions of the balsa, and it never looked like it soaked in at all. It also cured much more slowly than expected.

All I can think of is that it was cool that evening (though quite comfortable in short sleeves), whereas it may have been a few degrees warmer when I CAd the front of the body tube and the back of the motor mount tube. Does temperature have this dramatic an effect on thin CA on porous surfaces?

 

[dbarrym]

Were you using a fresh bottle of thin CA? To fully wick into the surface, it must be the thin CA - and not syrypy like it gets when old or if left sitting too close to a bottle of CA kicker...

Also, it won't soak in through the glassine coating on the exterior of body tubes.

barry

 

[lamart72]

CA needs moisture or humidity to cure quickly. I ran into this problem recently with the TARC team I am mentoring. The High School is air-conditioned ( with very low humidity) and the students had a hard time getting the CA joints to cure. Even after 10 minutes, the medium CA was still tacky. Touching the joint with a moistened paper towel cured it very quickly.

Lamar

 

[TeenRocketNerd]

Just curious, but would super glue work in place of CA in low/ low mid power? (not the gel super glue, the really runny stuff) Just wondering, I thought that it might.

 

[m85476585]

It's essentially the same thing.

 

[maxpower]

I practiced on a scrap piece of bulsa.

If I didn't spread it around it bubbled when it cured. This takes more sanding to get it smooth.

I then tried spreading the CA with wax paper (that is what I had handy) and it came out smoother but still needed sanding.

But...

Since CA doesn't actually make the bulsa stronger, just slightly more resistent to dings, I wonder if it is worth it.

I have also used sanding sealer. I like working with this more as it is easier to work with but it takes several coats and isn't any better at making the bulsa better ding resistant. It does make the bulsa finish real smooth.

Thanks for all the tips!

 

[dbarrym]

Probably based on my original post.

A few years ago someone had done empirical testing for the model aircraft community (can't remember the source or I'd post it, probably a column in a model magazine) and found that surfacing thin balsa with CA increased crossgrain (torsional) stiffness but added very little to the actual breaking strength.

Try it yourself - take two pieces of identical shape and size balsa (ideally cut from the same piece of balsa stock, with as close to identical weight and grain type as possible) and test them to breaking using graduated weights. The CA soaked wood will deflect less but finally fail at essentially the same loading.

It does help stiffen fins but since it is very brittle, it is more subject to dings (surface cracks) where balsa without the CA coating will dent. It's worth doing (I use this technique on thin tail surfaces for HLG's and RG/BG's, as well as all exposed edges of paper body tubes and transitions) but don't expect to replace ply fins with CA coated fins.

Barry

 

[Countdown Hobbies]

I have used both thin (Bob Smith Insta Cure) and medium (BS Maxi Cure) on balsa, the medium does leave a bubbly surface when I do it but with either you do have to sand for a nice finish.

I actually found that with the medium a nice thick epoxy brush works well for spreading and once it dries sandpaper and primer smooth it out well.

Thin CA should soak in and make it a bit easier but you still will have to sand it for a perfect finish.

Some balsa will soak better than others too so it is really different everytime.

 

[cjl]

I have found that depending on the rocket, the stiffening effect can be either beneficial or detremental to the fins strength. The rockets that I've stiffened this way all seem to hold up better in flight, without exception. However, they also all seem more likely to break on impact/landing. So, if you need strength in flight, use it, but if you need strength on landing, don't.

 

[dbarrym]

I'm curious because the CA soaked threaded holes held up better than the non-CA soaked in the retainer tests.

Absolutely - I do the same, even on threaded holes in ply. It makes sense, as you are "hardening" the edges of an otherwise very soft material. Balsa is a very porous material and CA is fairly hard.

At any rate, it is a good techique to use for multiple reasons, I wouldn't get too concerned about it one way or the other.

Barry

 

[RimfireJim]

Originally posted by lamart72
CA needs moisture or humidity to cure quickly. I ran into this problem recently with the TARC team I am mentoring. The High School is air-conditioned ( with very low humidity) and the students had a hard time getting the CA joints to cure. Even after 10 minutes, the medium CA was still tacky. Touching the joint with a moistened paper towel cured it very quickly.

Lamar

A trick I use to accelerate the curing on surfaces such as fins is to breathe on it, with the same technique as used to fog a cold window or mirror. The moisture in my breath is enough to kick the CA.

Just remember to inhale first well away from the CA!

 

[Mad Rocketeer]

Originally posted by dbarrym
Were you using a fresh bottle of thin CA? To fully wick into the surface, it must be the thin CA - and not syrypy like it gets when old or if left sitting too close to a bottle of CA kicker...

Also, it won't soak in through the glassine coating on the exterior of body tubes.

barry

The bottle had been first opened about a week and a half before, and the CA performed as expected at that time. I applied it to the inside (non glassine coated) surface of the paper tube and to normal untreated balsa. The only bottle of kicker I have has never been used and was stored at the other end of the house and had never been stored in the same box or been within 30 feet of the CA at any point since it left the hobby shop. The CA I used was Bob Smith Insta Cure, which is marked "Super Thin" and "1-3 Sec." It was at least as thin as water, but it didn't soak in at all. It just puddled on the surface and took a long time to cure, even when spread out. Weird. I wouldn't want it to cure before it soaked in, as I was going for stiffening, not filling.

 

[m85476585]

It's normal for it not to cure in a puddle, but it should soak in. Did it soak into the tube?

You must have a defective bottle or something.

 

[North Star]

Originally posted by Micromeister
Make sure your in a well ventelated area and never keep your face..Nose over the work. The towel mop will sometimes "smoke" but i've never had one actually ignite.

I'm surprised no one has come up with the 'tear gas' warning, Micromeister rightly warns about ventilation and proximity of face.

Nearly every time I use 'hot' cyano - the thin stuff commonly used for what we are discussing here, I get the smoke effect. If I'm indoors my eyes water and sting. I have used the 'odourless' type but it more expensive. You don't get this effect with small ammounts of cyano - its with the larger area jobs such as laminating or fin strengthening.

 

[Mad Rocketeer]

Originally posted by m85476585
It's normal for it not to cure in a puddle, but it should soak in. Did it soak into the tube?

You must have a defective bottle or something.

It's kind of an enigma. The "puddle" was a shallow one, as the CA spread out OK and was quite thin. It just acted like it was on a non-porous surface. The same stuff worked exactly as it should have just a few days before. I'll just chalk this one up as a mystery. I might try it again on the inside of the tube, but I've already primed the fins now. I used Kilz primer for the first time, but that'll be another thread.

 

[Mad Rocketeer]

I tried the CA on the tube again, and it acted the same. Just sat there until I shook it off the tube. I tried it next on some scrap balsa. Same deal. After a few minutes, I pressed a second scrap over the first and squeezed. They bonded, but a few minutes later I poked a fingernail between the pieces and popped them apart easily. Once apart, there was no visible evidence of the CA, so it must have soaked in. Not too inspiring.

 

[maxpower]

I am not having a whole lot of luck with the CA method either. In my case, it appears to soak in but it creates a rough surface and after sanding there appears to be little evidence of having put CA on the balsa. I have good luck with sanding sealer. Although I don't believe it is as good as CA for creating a ding free surface but it does appear to be better than nothing. The biggest negative is that you need several coats and it takes overnight to dry for each coat.

 

[dbarrym]

Originally posted by MaxPower
In my case, it appears to soak in but it creates a rough surface and after sanding there appears to be little evidence of having put CA on the balsa. (snip) The biggest negative is that you need several coats and it takes overnight to dry for each coat.

The balsa should be finish-sanded down to 320 grit before applying the CA, or you'll just raise the surface grain, which is much now much harder to sand! I rarely find that I need more than one coat.

If the CA does not kick off (at normal room temps and humidity) within 30 seconds or so, you've used too much. CA cures through a catalytic reaction and it needs to reach a certain critical mass of CA to moisture exposure before it 'kicks' - thick puddles can take a long time to do this.

The best CA for this purpose is the thin type, ideally a fresh bottle; thin CA that has gotten thick with age or moisture exposure will not wick into the wood and thus takes forever to cure. Remember that CA flows into wood through capillary action and if it's thick, it's kind of like pumping syrup through a straw.

Not an ad, but I stick with Zap brand CA's (the pink bottles for thin, green for thick). I have found that it is consistently better quality and seems to last longer in the bottle as well. I've used pretty much all of the hobby CA's but always come back to Zap. (Though I wish the original "Hot Stuff" was still around!)

 

[maxpower]

Good information.

I am using thin, Hobby Lobby brand. It is relatively new. I went with HL because I had a 40% off coupon.

I am working on another rocket at the moment and I will try sanding it with 320 before using CA.

Thanks again for the information.

 

[m85476585]

Originally posted by MaxPower
It is relatively new.

There's no way to tell how long it's been sitting on the shelf. Great Planes brand has an expiration date.

 

[Mad Rocketeer]

Bob Smith has always worked fine for me up until this bottle, which worked briefly then quit. I dunno what to make of it. I've had the stuff set up in the bottle more than once, which is why I buy only the 1/2 oz. bottles now, and I'm careful how I close them (puffing them out first) and what touches the tips. Ah well.