Balsa to Super Smooth: Putty vs. Sanding Sealer vs. ?

[Aksrockets]

I just started using bondo glazing and spot putty. That stuff is a dream to work with. Smooths on nicely, dries quickly, sands easily and leaves a beautiful, slick finish!
It's also dirt cheap.

Alex

 

[SSenesy]

Just an FYI; there are two different Bondo spot putties.

Bondo Glazing and Spot Putty is what everyone has been describing here. It's a single part putty that works well to fill spirals, grain and small imperfections.

Bondo Professional Glazing and Spot Putty is a two part system that works like epoxy. It comes with a tube of white putty and a small tube of red hardener. It is stronger than the single part system and can fill much deeper voids. The only drawbacks that I've found is that it dries EXTREMELY quickly and costs a bit more. You usually get 5 minutes or less to use it. High temps speed the drying time.

I prefer the 2 part putty because it can be applied pretty much as thickly as you need and it will still cure and have strength.

YMMV.

Here's a 5.5" Polecat Hawk that I've applied the 2-part Bondo over the colloidal silica fillets but haven't sanded it out yet.

 

[Marc_G]

A while back somebody posted a suggestion that I tried and really liked. I'm sorry I can't remember his name (Its been too long since I have been able to build/repair anything).

He mixed the Bondo putty and Minwax wood hardener 50:50 by volume and mixed well. The result was easy to paint on, dried quickly and sanded easily. Most impressive to me was the appearance. The result was like plastic.

I started doing most of my projects this way and it worked for me very well.

Ah! That sounds like the Teamwork Filler I described back in late 2010 or early 2011. Worked like a charm for me on several rockets. IT was my preferred way to do things for maybe a half dozen birds. Then I discovered that sealing fins with CA, followed by glazing and spot putty (straight or sometimes thinned with lacquer thinner if needed) worked just as well, gave equally smooth results, but took hours instead of days from start to finish. Lots of folks mentioned they liked to let the wood hardener or mixes including it plenty of time to dry/cure (as in, days to weeks to even months for thick wood like nose cones).

For the record I now seal with thin CA, occasionally topcoat with thick CA on really grainy balsa, and take one of two routes:
-If the grain is minor, I just hit it with several coats of Rustoleum auto filler primer, sand, and call it done, or
-If significant grain remains, use the spot putty or CWF depending on my mood, sand, filler prime, done.

I'll note that the CA sealed wood is quite hard so sanding after the CA step can be done quickly with good pressure because the filler primer, spot putty, or CWF are all very soft compared to the sealed wood. No worries about oversanding.

Marc

 

[Green Jello]

For those who use sanding sealer...what's the best brand, where do you get it, etc.?

 

[LW Bercini]

Bondo Professional Glazing and Spot Putty is a two part system that works like epoxy. It comes with a tube of white putty and a small tube of red hardener. It is stronger than the single part system and can fill much deeper voids. The only drawbacks that I've found is that it dries EXTREMELY quickly and costs a bit more. You usually get 5 minutes or less to use it. High temps speed the drying time.

I prefer the 2 part putty because it can be applied pretty much as thickly as you need and it will still cure and have strength.

I started out with the 2-part putty but the odor literally made me sick.

 

[Green Jello]

Here it is. Found it at O'Reillys. Autozone didn't carry it.

 

[SSenesy]

I started out with the 2-part putty but the odor literally made me sick.

It IS some stinky stuff. I was doing a build last winter and thought I could get away with a quick application in the basement while my wife was out. It spread throughout the house. When the wife got home, she was NOT happy. It ended up costing me a nice dinner and night out so she didn't have to suffer with the smell. When I use it now, it's usually the last thing I do in the day and then I lock the garage up and go back to the house.

 

[Bone Daddy]

Part of the reason there are so many techniques is that the quality of balsa wood used in kits varies tremendously.
Even from lot to lot of the same kit.
Everything from grain the size of sidewalk cracks to almost baby butt smooth.
There was a time when the balsa in Estes Wizards looked like they needed a shave
before you could even start to work with them.

My solution is to just say no to balsa fins.
I replace them with basswood.

Much easier to get a nice smooth finish.
More dense so they don't ding easily.
No appreciable weight penalty.
No appreciable added cost.

Sand smooth to remove the uneven areas.
Shoot a couple of shots of high build primer with some sanding in between and bingo, glass smooth fins.

I've also become enamored with trim Monokote.

Pic 1 Wizard Balsa
Pic 2 Wizard fin in basswood
Pic 3 Wizard Grande upscale fin basswood
Pic 4 Wizard Much Grande upscale basswood with yellow trim Monokote

My guess is that after trying basswood you may never look back.

Bones

 

[Reddwarf]

Great thread. Some newbie questions. 1. Where does one get sanding filler? I recall using that as a youth but the hobby shop guy had no idea what I was talking about. 2. What is CA? I know glue, but what brand(s)? Where does one buy? 3. The bonds idea sounds good except for the smell issue. I am confused, is it one product or two?

 

[Green Jello]

Great thread. Some newbie questions. 1. Where does one get sanding filler? I recall using that as a youth but the hobby shop guy had no idea what I was talking about. 2. What is CA? I know glue, but what brand(s)? Where does one buy? 3. The bonds idea sounds good except for the smell issue. I am confused, is it one product or two?

1. Do you mean sanding sealer? If so, most hobby places carry it. The Aerogloss brand is one of the most common:
https://www3.towerhobbies.com/cgi-bin/WTI0001P?I=LXAU64&P=8

2. CA = cyanoacrylate......commonly known as super glue or crazy glue. That fun liquid stuff that sticks your fingers together. You can buy large quantities in different thicknesses at places like this:
https://www.hobbylinc.com/cgi-bin/s8.cgi?str_s=Cyanoacrylate&x=0&y=0&ss=1
Having some thin CA around is really useful for rockets.

3. The Bondo Glasing and Spot Putty is one product:

 

[LW Bercini]

... 3. The bonds idea sounds good except for the smell issue. I am confused, is it one product or two?

3. The Bondo Glasing and Spot Putty is one product:
View attachment 94823

Reddwarf,

The stuff Green Jello showed you oint: is the good stuff. It is a one-part putty (one tube, no mixing), and although it does have some smell, I can tolerate it just fine. I use it and love it. It is the odor of the 2-part stuff (i.e., a base with a separate hardener that you mix together) that made me sick.

 

[Green Jello]

I just used it for the first time today (indoors) and I didn't find it any worst than the other stuff we use like CA, spray paint, etc.

 

[Reddwarf]

Another nebie question. I see the AreoGlass (thanks for the link). I also found Sanding Sealer at Lowes 1qt for 10 dollars. Another can by Minwax a bit more pricey. Any reason those would not work? Seems a lot less expensive, although granted more product than I need!

 

[luke strawwalker]

Great thread. Some newbie questions. 1. Where does one get sanding filler? I recall using that as a youth but the hobby shop guy had no idea what I was talking about. 2. What is CA? I know glue, but what brand(s)? Where does one buy? 3. The bonds idea sounds good except for the smell issue. I am confused, is it one product or two?

Filler-- there are two schools of thought... basically it boils down to you can do it the easy way or the hard way... the easy way is go the hardware store or indoor lumberyard store and get a small tub of "Elmer's Carpenters Wood Filler"... the best way to use this stuff I've found is to put a dollop in a baby food jar and add a few drops of water and work them into it with a 1 inch paintbrush until you get it the consistency of yellow hot dog mustard... then brush the stuff on, let it dry, and sand with 220 grit followed by 400 grit. Papering the fins adds a lot of strength and does the filling for you, totally eliminating the filling and sanding on the fins and adding a lot of strength, but you'll still have to fill and sand balsa nosecones and transitions. The "hard way" (IMHO, though others disagree) to fill fins is to go to the Hobby Shop or order sanding sealer "dope" online and brush multiple coats of the foul-smelling stuff onto the fins and sand them multiple times... CWF (carpenters wood filler) is just SO much easier IMHO and does JUST as good a job if not better IMHO...

CA-- cyanoacrylate glue... AKA "super glue". Available in bottles from the local hobby shop, Hobby Lobby, or from online vendors. In my experience, the stuff is VERY limited in its usefulness in rocketry construction-- it becomes brittle with age, and while extremely strong in TENSION, it is VERY weak in "shear" planes, IE, the impact forces that occur when the rocket's fins hit the ground first on landing. CA is best used to "soak" into balsa nosecones and transitions to "harden" them somewhat before filling and sanding... other than that, and perhaps a LITTLE detail work, I don't recommend CA for anything. CA soaks into balsa and seals off the "pores" in the wood, which then makes it IMPOSSIBLE for yellow wood glues or white glues to get a good bond with the wood and strongly attaching the fins to the rocket. THE strongest method for gluing balsa fins onto a rocket is the DOUBLE GLUE JOINT using plain old yellow carpenters wood glue. Apply a thin layer to the fin root and the tube on the fin attach line. Allow to dry 20-30 minutes while you do some other step on the rocket. Apply a second thin layer of glue over this first layer on the fin root, then carefully align the fin with the rocket tube, and press them together-- hold 20-30 seconds, and PRESTO the fin will "lock" in place like you glued it with super glue, but with about five times the strength... you can glue all the fins on at once, fillet them, and you're done. MUCH stronger and FAR superior to super glue in every respect...

later and good luck! OL JR

 

[cwbullet]

I have used wood glue before. It works, but takes quiet a bit of sanding. I am going to try spackle and drywall plaster. I really like fill N Finish and Icing. Bother work and are very light.

 

[AfterBurners]

yep can of worms is now open. I just used the Aero gloss dope on my balsa nose cone and turned out nice. I used 2-3 coats of balsa filler primer followed by 2 coats of sanding sealer. It turned out nice. Gotta love the smell too. No chance of warping fins. I say experiment with different techniques and pick one you like the best and stay with it. Ideally you want to be able to get nice results regardless of what method you use. You could also skin the fins with copy paper.

 

[RocketMike]

When papering balsa fins, does one round the leading edge before papering or after? Also have always wondered how much value there really is in rounding the leading edge on a sport rocket anyway?

 

[AfterBurners]

For those who use sanding sealer...what's the best brand, where do you get it, etc.?

Aero Gloss works well...

https://www.brshobbies.com/catalog.php/BRSHobbies/dt94162/Building_Supplies

 

[AfterBurners]

When papering balsa fins, does one round the leading edge before papering or after? Also have always wondered how much value there really is in rounding the leading edge on a sport rocket anyway?

You don't have to round them. You can use two separate sheets and then trim the paper close. Use some CA after it dries on the edges except for the root edge to seal the paper and keep it from pealing back.

 

[GDJ]

When papering balsa fins, does one round the leading edge before papering or after? Also have always wondered how much value there really is in rounding the leading edge on a sport rocket anyway?

It can add substantially to the speed and altitude of the rocket flight.
Going to a symmetrical shape airfoil can add more speed and altitude.

Personally I've used this shape many times and it seems to work very well.

 

[luke strawwalker]

When papering balsa fins, does one round the leading edge before papering or after? Also have always wondered how much value there really is in rounding the leading edge on a sport rocket anyway?

Yes, you airfoil your fins first...

Airfoiling the fins can DRASTICALLY cut drag on the rocket... which directly translates into increased altitude. If you want to maximize performance, you need to airfoil your fins. If you're willing to accept lower performance, airfoiling isn't necessary... It's up to you...

Later! OL JR

 

[bill_s]

If you look at drag coefficients of shapes, subsonic, even the slightest rounding of the leading edges has much less drag than squared off shape, maybe a third more drag than a perfect parabola. This doesn't help trailing edges though, it needs to be a gradual taper to cut drag which weakens the edge, so I'd use a slight taper squared off or just leave it square for strength.

 

[luke strawwalker]

If you look at drag coefficients of shapes, subsonic, even the slightest rounding of the leading edges has much less drag than squared off shape, maybe a third more drag than a perfect parabola. This doesn't help trailing edges though, it needs to be a gradual taper to cut drag which weakens the edge, so I'd use a slight taper squared off or just leave it square for strength.

Well, if you go with the true airfoil shape with a LONG teardrop shaped tail... BUT, even a "short" teardrop shape will be a LOT more efficient than leaving the back of the fins squared off...

Later! OL JR