Filling Balsa Grain
 

Hi guys. Just thought I'd throw in my 2 cents worth on this topic, since there seems to be plenty of interest on the subject. I've read in one of the forums about techniques where people use white (or yellow) glue to fill and seal the grain. These glues can be sanded only with difficulty and I have found them more useful for stiffening or reinforcing paper areas on a rocket (such as shrouds) than for actually filling the balsa grain. Also, these products are water-based, and all water-based coatings will tend to warp fins.

I use a product called 'BONDO' glazing and spot putty which is used in the automotive refinish business. Because it is not water-based and is readily thinned with acetone or toluene it is great for model rocketry finishing. It is completely compatible with dope (and, in fact, I thin it with dope to get a more workable product, but more on that later) and so it makes a great surface for your final sanding sealer step, if that is part of your finishing technique.

I should point out that this material is available at your automotive and hardware stores under many different brand names. Just be sure to get one that is NOT water-based (I'm not even sure that water-based ones exist, but if they do, stay clear of them for model rocketry applications). Also, I should point out that this product is not compatible with white or yellow glue, even when dried on your rocket (will not adhere to it), so it's important to keep your glues fillets on your fins small and neat for the best possible finish.

The first thing I do is to dilute the product approximately 50/50 with dope. You can use clear or white, or for that matter any color you wish, but darker colors will be harder to cover with your primer coat. In diluted form it flows more readily into the balsa grain. The application technique I use is to mix it to a uniform consistency and then apply it with a stiff piece of balsa (for this, a short piece of balsa stick about 1/4"x1/4" in cross section works well). It's important to use balsa and not a harder material like a wooden or steel ruler, because these can put digs into the surface of the relatively soft balsa. I apply a cover coat to the fin as quickly as possible (it dries fairly quickly) and then use the stick to scrape the surface of the fin hard (the harder the better...........support the fin while you do this). For curved surfaces such as nose cones I use my finger to work it into the grain. A rubber glove is advisable to avoid contact with the skin, and all of this work should be done in an area with good ventilation (outside if possible). The scraped filler can be returned to the bottle. This technique minimizes the amount of sanding you will need to do in the next step. Because there is a fair amount of volatiles in the diluted product, it shrinks down upon drying, so after the first sanding you will probably see that there are still depressions where the grain is. But fear not, sanding is very quick and easy, so I sometimes use up to three coats with sanding between each.

A word about sanding..............I find the sanding sticks my wife and daughters use to sand their fingernails work great! They have a coarse grit on one side and a somewhat finer grit on the other. Best of all, they are flat and stiff, so you can avoid the wavy surfaces you sometimes get when sanding with sandpaper.

I always finish off with several coats of sanding sealer, sanding with a very fine paper between each. The result is a substrate for your primer or topcoat which can be almost as smooth as glass. While such treatment will add a bit of weight to your rocket, some experts maintain that performance increases from the smooth surface outweigh decreases from the added weight. And the bonus is a rocket that looks great! Incidentally, the most fastidious of modelers will apply a filling and smoothing technique to hide the spirals of the body tube as well.

Finally, a word about the butyrate dope I use to dilute my BONDO. It is still being produced in the US by at least one manufacturer and so is available in hobby shops. It is also available under the tradenames 'Testors' and 'AeroGloss' and appears in auctions from time to time on Ebay. More info in one of the other threads on butyrate dope. As a topcoat for your final finish it is, in my opinion, unsurpassed. It is mostly volatiles, so it gives a thin, light hard finish on your rocket. I used the above-described technique for the nose cone of my Astrobee 1500 (pictured below) which was filled with ruts and crevices in the balsa when I removed it from the kit. When dry, dope can be overcoated with Krylon clear or another topcoat of your choice. But always check for compatibility before you spray as you don't want to ruin the beautiful finish on your masterpiece. Incidentally, while Krylon and other plastic sprays can be applied over butyrate dope, the reverse IS NOT TRUE. Dope will mar and ruin a plastic-based finish.

I would be interested in hearing about anybody else's experience in finishing.

Joe

I always say if it works keep it. I have tried many techniques, and the one that works best for me is multiple coats of Aerogloss sealer. I have tried other techniques and they do work, but for what ever reason they did not fit my needs.

the wife would put >me< outside if I used this stuff. so I have to stick with the non-stinky alternatives. f-n-f, weldbond , ect...

For me, if the balsa filler of choice is not "stinky" with a large Organic Solvent content, it just is NOT right....I don't like any of that water-based JUNK.
Good Ol' Aero Gloss Sanding Sealer and Balsa Fillercoat were good enuff during the golden era of model rocketry, and they work great now (if you cand find NOS)
The only time I ever use fill-n-finish is when I have some sort of a huge gap that needs to be filled in....If "Bondo" was not so darned heavy, I would use it instead.

The best method is the one that works for you!
I use to use Aerogloss, works great. But I sometimes have a hard time finding it.
Now I'm using a product I bought a Lowe's, it's called Deft Lacquer Sanding Sealer. I buy it by the quart. It also comes in a spray can, but I've never tried that. The quart is very economical. It's fast drying, 10 minutes. And it's easy sanding. So in a nut shell it's cheap, fast drying, and easily sandable. You can't get any better than that. ;) You can even buy it by the gallon if you want. Three coats, sanded after each and you can't see any wood grain. :D

Please don't hate me for asking this...

Other than aesthetics, is there a need to fill the grain? That is, if one was ok with the "painted wood" look, could one simply sand it smooth, paint, and fly, without a noticable increase of wind drag or what not?

Just wondering...

-Tim

I agree, but for some reason I like filling grain. When I built rockets in the late 70s, nobody filled the grain. The balsa was sanded smooth and one top coat was added. The fins were sanded smooth again and one more coat of paint. No primer.......The grain did show up close, but the rocket still looked nice because of the good build, paint and decal job.

I don't use sanding sealer, I just prime and sand (and repeat...) but I have to say I prefer the smooth look. I really don't know how much difference it makes performance-wise.

I guess it depends on what you want from your flying model rocket, tfischer. If you're just flying it, then I guess it might seem silly to do all that work just for you local rocket eating tree (who I hear also has a passion for kites, gliders, and RC aeroplanes). It doesn’t care how good it looks going down. :D What was once a pretty boy ain’t so pretty by the end of summer.

If you're never going to fly her, then all those little details make more sense given the amount of time it will spend sitting on your self for display. No sense rushing a 'Show Queen'. You do you best job for that rocket because you know it will never fly.

On the other hand, I want it all: a rocket that looks equally as good on the shelf as it does in the air. All the rockets I build must fly, but I also want them to look like they came straight out from the flying model rocket catalog. :rolleyes:

Smooth, glass-like fins just make the paint and the decal application easier. Untreated fins make the decals stick out more like a sore thumb, and are harder to apply. Of course, with all the 'Peel and Stick' decals that come from Estes today, it may not matter so much.

Decals are suppose to blend in with the paint and conform to the shape of the model's edges, but 'Peel & Stick' decals sit on top of the paint and have a noticeable clear plastic film. Good waterslide decals blend in with the paint and will not detract from the model’s appearance but enhance it. Normally, I use decal sol and decal set solutions to move the decals around and help blend them into the paint. Decal sol and decal set are useless on vinyl 'Peel & Stick' decals, which will cause them to distort and diffuse on the surface rather than blend in.

But to get good decal adhesion, I need to glass the fins--first--before I paint them. Any other way just doesn't cut it.

I find that using Bondo glazing and spot putty, balsa fillercoat, and sanding sealer are the best methods for me. Pacer's Z-Poxy Finishing Resin also does a marvelous job but takes much longer to cure. And there's nothing wrong with just using auto-primers and sanding until smooth.

I still, yet, need to try the Elmers Fill'n Finish method. They just don't sell that anywhere around here. :(

Just to clarify, my question wasn't "why are you crazy people spending all that time/work?" but "should I really be doing that, even on my basic "get it built and flying" models?"...

I understand that the filling should be done for the models to look their best. Just wanted to make sure that it wasn't something that shouldn't be done, even if looks were secondary to flying (and time)...

Thanks for the info!
-Tim

The reason why many rockets instructions have some sort of fin filling recommendation is because if you don't fill the fins with a light weight wood sealer of some kind, the fins will absorb most of the topcoat paint. As balsa can absorb 5 times its weight in water, that can be a lot of extra paint and added weight. Sandable primers do a basic job of sealing the balsa wood but aren't as good a sealant as sanding sealer or balsa fillercoat. You need at least two coats or more of something to prevent the balsa from soaking up all that topcoat paint, or your rocket will weigh more than the recommended weight on the package, and then I would only fly that rocket with shorter than recommended delay charge.

But that's entirely up to you. If you think it's too much work, then don't worry about. Your rocket can still fly as well without filling as with filling the fins, as long as you don't use too much paint. Stability should be about the same. As for looks, well... you get what you put into it. ;)

This is one of those subjects where everyone has a favorite method of finishing the balsa, and every one of these methods has its pros and its cons. I've tried many different grain-filling ideas, from talcum powder mixed in clear dope, to Fill-N-Finish, to Kilz, to water-based polyureathane, to just painting them with enamels. Currently, my method-of-choice for fins involves none of these...

...well, not directly, anyhow...

I'm becoming quite fond of a very simple technique which takes a piece of self-adhesive mailing-label stock, cutting it to the outline of the fin, applied and pressed in place on both sides, then sealed around the outer edges with thin CA. To date, I've had none peel up (delaminate) from the balsa, and when you hit it with primer, it's sealed. The paper is lightweight, and completely hides the balsa grain in one step.

This method has the one cardinal advantage of not contaminating the room with foul solvent odors (other than what you get from CA and accelerator). You can get sheets of the label stock from any office supply store (Office Max, Office Depot, Staples), and it doesn't require the most expensive brand. I'm using the "house brands" with equal success. You can even run a sheet through the printer first and lay out your cutting patterns to get an absolutely accurate outline. Just run your #11 along the lines with a steel ruler for guidance.

I try to think of this as "modern classic" construction. It's a composite lay-up, like applying glass cloth and resin, just cleaner and less noxious (less ob-noxious :D ) to your housemates. The results are more than acceptable to this old fart...

I don't know who came up with this idea first, but it was Jay "Tau Zero" Goemmer who (I think) got me started.

"You *can't* be Sirius." "--Oh, *yes* I can..."
 

I'm very happy to admit that it was David J. "Sirius Rocketry" Miller of Neenah, Wisconsin fame:

http://www.rocketreviews.com/review...terrogator.html

;) :D

No contaminating the room with foul solvent odors ? :confused:
C'mon now.....
I prefer to refer to the scent of Aero Gloss Dope (or Sig, Brodak, etc.) Sanding Sealer or Balsa Fillercoat as a PLEASANT AROMA, NOT a "foul odor". :p
This step in the build contributes to the "nostalgia" of being a BAR.
Thankfully, my wife finds the scent of drying butyrate dope pleasant as well ! :D
Most of the time I actually apply my Sanding Sealer and Balsa Fillercoat to fins/nose cones/transitions where I usually build/work on my rockets, the KITCHEN COUNTER, which my wife does not even bat an eye at....probably because she has an entire room of the house dedicated to her scrapbooking activity.

Tim,

To make a long answer short, fillin' and sealin' are not required if you're just out to fly 'em and don't care how high they go or how good they look.

Tim,

I seem to recall reading an Estes publication (admittedly, long ago, probably an issue on Model Rocket News) that dealt with the subject of finishing model rockets. In the article it stated that the performance plus of a smooth finish (i.e. lower air friction/resistance) outweighed the performance minus of any additional weight as a result of the finishing technique. So a really pretty rocket will probably outperform its scruffy cousin. Why heck, in the old days Estes even sold little containers of rocket wax to polish up your rocket and get the finish even smoother! Yep, believe it or not!

Joe

Heh, I can see it now. Hey Bill, what are you doing over there with that magazine? Oh, just polishing my rocket. :D

I prefer "Big Bertha" style flights -- low, slow, I can see the rocket all the way up, I do not have to walk a mile to get it. I preferred this type of flying when I did high power. Fly to 20K feet? Are you insane? I want that rocket to return. And for all the money I spent on that reload, I wanted to enjoy SEEING every Newton-second of burn!

Hence, any potential performance losses due to a heavy finish, are, in my opinion, a blessing!

I want the rocket to look great, and for me that drives me to fuss over the littlest imperfection.

Call me crazy! I'll agree with you.

:)

LMAO!

I agree with you. When I did my bigger HPR projects, I kept the altitudes down between 2700 ft and 5800 ft. My biggest, a 12 ft rocket, was still easy to see even at a mile. The first flight of that Aerobee was only about 2700-2800 ft. and was the best flight of the week at LDRS (IMHO, of course) :D

Craig,

Many years ago (I shudder to think how many) I used your technique for finishing my fins with mixed results. I started by simply putting a thin coat of white glue on the fin and then laying on the paper which I had previously cut to shape. I quickly learned to cut it a bit smaller than the balsa fin after having to sand the curve into the leading and trailing edges (a LOT more difficult with the paper and glue there) on the first set I fins I produced this way. Fins made this way often warped slightly, especially when the fins were thin. So I then went to peel and stick paper (Estes PRM-1), also with mixed results. Although this solved the warpage problem, I sometimes got a bit of delamination at the fin edges and 'bubbling' on the fin surfaces. So I was never totally happy with the result, and never perfected the technique further to overcome these problems. Still, I continued to use the method for a while for two reasons..............the balsa grain was completely filled with little effort, and the fins were VERY strong.

Joe

fin grain needs to be filled, lol ...

when starting out, of course.. I never sealed balsa, then started to use Aerogloss, usually 1-2 coats to seal the wood, but never filled all the grain - hence my vintage builds looking not as perfect.

when I got into HPR, when spending lots more time ( and DOLLARS ), figured they should look nice ( plus plywood seals easier, lol ) , so started making them real clean ( huge influence here by one of my mentors, thnks RON :)

now into my nostalgioa phase ( I'm NOT a BAR, never left, just been through different phases ) - I strive to make every one more perfect than the last - meaning perfect sealed fins, sanded fillets... never used to fill spirals, but have to admit, my last 3-4 builds / painted projects got filled spiral grooves - including the CHallenger 2 ( originally a 'no painting required' bird :)

but, if you prefer raw grain and ' going to loose it anyways' flights - been there, done that ( but send me your unused decal sheets from your naked birds, might have use of those :D

just my 27 1/2 cents ( inflated for modern composite motor costs )


~ AL

Nowadays I like to fill the grain ( in the eary days I didn't care).
but I never understood using a "sealer" to fill the grain, thats alot of work !

If a person just likes the wood sealed than that sounds likes the way to go.
using a filler like f-n-f gets it done in one application, thats why I like it, lees work , great results. even easier for me is paper laminating, I do that alot now, easy and strong.

I need to conserve what braincells I have left anyway ..

no one way is right or wrong just each person's preference

I often paper fins using standard copier paper and a very very thin layer of yellow glue. I haven't tried it on very thin balsa yet, but on the thicker stuff I've not seen any warping.

Water-based glues will do that; especially on thin balsa, like 1/16" sheet. I'm also into RC aircraft, and tried laminating a thin balsa skin to a foam core with white glue. It was a disaster, until I found a better method.

Apply the glue in a thin layer to both the balsa and the paper. Allow to dry completely. Lay the paper onto the balsa, then take a Monokote iron and heat-press the paper onto the balsa. The heat passes through the paper to the glue, melts the glue slightly, then creates an impossible-to-peel lamination as the glue polymerizes. By allowing the glue to cure completely, you don't trap the moisture in the wood, which causes the warping problem.




I don't bother doing this anymore. What I do is cut the label sheet to the same outline, press it down, then apply the thin CA to the edges. The CA wicks down into the balsa and sets up rock-hard. Now is when I take the 220-grit to the edges and round them just enough to remove the roughness of the CA.

Sometimes I don't pre-cut the label. I just take the pre-cut balsa fin, lay it out on a piece of label stock, then trim away the excess. The sanding takes care of any remainder.



One thing to keep in mind is the improvements made in adhesives over the past 35-40 years. The current variety of label stock is far superior to whatever was available in the late '60s and early '70s. Go grab some label stock and try the technique again. If you don't seal the edges, however, the label will peel. Sealing the edges with thin CA does double-duty -- it attaches the label edges down firmly, and hardens the edge of the balsa.



It's a laminate, or composite, structure; very much like 1/16" thick balsa plywood (or MDF board). It's going to be stronger than the balsa by itself, no matter how it's filled.

I've been using CA to seal fin edges for a while now, and I have to say that it works great!

OK, guys. Time to give the old paper laminating method a try again. You've got me convinced! Especially because I agree that the adhesives used on paper label stock is much better than it used to be. By the way, the dry glue/iron-on method is ingenious if it works well............can't wait to try that! One question though.............what is this 'CA' stuff that you guys use?

Joe

Cyanoacrylate better known as Crazy Glue. Any hobby shop will carry the stuff.

Watery-thin Super Glue. Comes in a Teflon bottle, always hardens in the cap before it hardens where you want it to, smokes when you drop it onto balsa sawdust, turns into a hard foam if you hit it with accelerator...

Frustratingly useful stuff...:D

CA..............cool, thanks, guys. I've used it many times, just not for rocketry.

Joe

Put a piece of plastic baggie over your finger, and you can rub the CA into the fin edge (but not the root edge, unless you are gluing your fins on with CA). Obviously, omitting the baggie will lead you to become one with your work... :D

I've come to the conclusion that any glue that shrinks when it dries, is not ideal for paper laminating. 3m spray does the trick for me.

Yep....that's what I use. Found it helps to tape down the side that's not to be sprayed so it doesn't blow away from the spray.... :eek:

Hmm. Interesting theory... but I suspect the reason it works so well for me is that it does, in fact, shrink. Recall I said I use a very very thin coat... the shrinkage pulls the paper down tight to the balsa.

But of course, do what works best for you. I know that my finishing results are pretty decent, and I break about half the "rules" I've read for finishing. And that includes only the "rules" that don't conflict with each other...

I haven't read the whole thread... But what I find excellent/easy/fast is tissue put on with sanding sealer dope. Never found a need to look further once I was taught that way, beautifully smooth (after a light sand) and much stronger. May be a bit old-fashioned, but it works!

I'm interested in knowing more about the tissue method. I built a number of control line models as a kid, and liked the finish. Do you just take the tissue to the edge, and sand it flush when dry, or actually overlap? I'd think taking it to the edge would be fine, especially with curved edges, where wrapping could be a pain.

Actually, wrapping isn't a pain. When the dope/sealer gets on the tissue (you apply it to the balsa) it easily follows the curves. It lays so well that overlapping still looks/feels even after a light sand. The increase in strength is amazing...well, to me it was. I didn't believe it before I tried it, it didn't sound like it would do much. The best thing is to do one fin and compare with a "raw" one to become a believer, feel how much harder the fin is to bend, and it still weighs almost the same. Overlapping on the tapered edges helps maintain that nice surface after the work you put into it too.

I put a layer of sanding sealer on after the initial tissue application has dried. Sand with 320/400/600, nothing coarse at least, it doesn't take much. I also make sure any big "faults" in the balsa are taken care of before the tissue, it won't hide them for sure, as usual prepping before the final finishing steps (tissue) gives best results.

The best is Japanese tissue, they say. I have always just used regular model tissue (Sig I think).

It seams like there are many techniques for sealing fins. I find the little effort it takes for smooth fins is worth it. I sealed and painted these fins today with aerogloss sealer and primer.

I had my opportunity to use aerogloss for the first time yesterday on the nose cone of my scratch build..And am sold on it :) Here is green wave waiting for the 2nd color to be shot on..Look ma no grain lines on the nose cone!:D

Pantherjon, That is good to hear, I know everyone has there opinions on Aerogloss. For me it has worked better than any other technique I tried. I have no problems sanding 1 hour after application and priming the same day. It also brings me back to when I use to build back in the late 70's early 80's.