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

Since I got back into rockets, I've been doing the fins by painting them with watered down wood putty and then sanding. It works pretty well, but I can't seem to get them to look 100% like plastic. I've seen that some of you use sanding sealer. Which is the better method? Is there a third way? For purposes of this question, I'm talking specifically about balsa fins.

Thanks

Green Jello said:

Since I got back into rockets, I've been doing the fins by painting them with watered down wood putty and then sanding. It works pretty well, but I can't seem to get them to look 100% like plastic. I've seen that some of you use sanding sealer. Which is the better method? Is there a third way? For purposes of this question, I'm talking specifically about balsa fins.

Thanks

Click to expand...

Unfortunately, you have asked what quickly becomes a passionate debate over individual philosophies. One person will say that "so and so" is the BEST way and another person will say that it is really the WORST way.

I recommend you do a series of searches in the forum regarding papering, using glues and putties, sanding sealer, FnF, primers, and spackles. Try out different techniques and see which one works best for YOU.

I'm sorry if this sounds like an evasive answer, but the reality is: finishing techniques are a personal choice, and as such can generate an almost religious fervor.

I would have to say that the smoothest, glassiest finish I have ever gotten with balsa fins is coating with thin CA, letting dry, then sanding. Since CA is essentially a liquid polymer, it ends up looking and feeling like plastic. There are a few caveats when using CA as a sanding sealer, however:

1) Apply in a WELL ventilated area. At the very least, next to an open window with a fan pulling the fumes out. Because this much CA will create a LOT of eye-watering fumes.

2) Use plastic as a barrier between the balsa and your skin. I have some polyethylene gloves that I use when doing this - they don't stick to the CA that well and I can use my gloved finger to smooth the CA over the whole surface so it soaks in. In a pinch, you could probably use a sandwich bag or something similar to apply and spread the CA.

3) Don't sand through the CA. Unlike sanding sealer, you don't want to remove too much of your CA - it needs to form a shell over the entire surface of the balsa. this is easier to do with nose cones than with fins - the curved surface just seems to invite sanded through spots.

4) Start with a medium grit sandpaper and work towards finer grades. Ultra-fine sandpaper will give you a glass-smooth finish.

I'm a big fan of Elmer's Wood Filler for anything that is wood on my rockets. As you've said, water it down so that you can easily brush it on. My procedure is as follows:

Paint on first coat of filler, and sand almost completely off. Just trying to fill the grain.
Paint on second coat, again sanding almost completely off
Spray on a coat of high-fill primer and look for major imperfections
Fill imperfections with undiluted filler, sand again
Prime again
Lather, rinse, repeat until you're happy with the finish
Wet sand with 400 grit to prep for the color

You've mentioned the 'plastic' look - generally, I'm happy at the primer stage if I don't see any spirals, grain or other imperfections. The rule of thumb is that anything you can feel with your fingers will show in the color coats. Getting that 'plastic' look when painting is a whole different animal. Now you're talking about wet sanding the color coat (or clear coat if you're doing 2 part), compounding and polishing. If done by hand, it can take hours and hours of work to get a perfect finish, even on a small rocket. It's possible to use machine tools on larger rockets, but you've still got a LOT of work ahead of you. If you go this route, just remember that it'll only look perfect till it's first flight

For clarification -
Elmer's Carpenter's Wood Filler or CWF isn't a sealer.
Sanding Sealer is a sealer.

Water and even wood stains will soak into a CWF filled fin.
Stains won't soak into a sealed wood surface.

Balsa surfaces filled with CWF aren't really sealed until a primer coat or color coat is sprayed on.

hcmbanjo said:

For clarification -
Elmer's Carpenter's Wood Filler or CWF isn't a sealer.
Sanding Sealer is a sealer.

Water and even wood stains will soak into a CWF filled fin.
Stains won't soak into a sealed wood surface.

Balsa surfaces filled with CWF aren't really sealed until a primer coat or color coat is sprayed on.

Click to expand...

My bad Edited and fixed - it was a bit late for me last night.

I've used Kilz as a combo filler/sealer before, but haven't been happy with the 'sand-ability' of the finished product. Generally, I just try to avoid spilling anything on the fins before fully primed. This hasn't been an issue with any of the rockets I've built so far, but I've got a 5.5" Polecat almost ready for finishing and there's so much wood on the fins that a sealer will probably be wise to prevent warpage.

SSenesy said:

I'm a big fan of Elmer's Wood Filler for anything that is wood on my rockets. As you've said, water it down so that you can easily brush it on. My procedure is as follows:

Paint on first coat of filler, and sand almost completely off. Just trying to fill the grain.
Paint on second coat, again sanding almost completely off
Spray on a coat of high-fill primer and look for major imperfections
Fill imperfections with undiluted filler, sand again
Prime again
Lather, rinse, repeat until you're happy with the finish
Wet sand with 400 grit to prep for the color

Click to expand...

I think this is what I'm going to go with. The primer I used last time wasn't fill primer, so maybe that will help. I have two questions though:

1. In that list of steps, where do you glue them to the body tube? After the second coat of filler but before the primer?

2. How do you avoid the watered down wood filler from warping the fins? I've just been working fast to get it on both sides quickly and then smashing them under heavy books overnight.

Green Jello said:

2. How do you avoid the watered down wood filler from warping the fins? I've just been working fast to get it on both sides quickly and then smashing them under heavy books overnight.

Click to expand...

I apply the wood filler evenly to both sides. When I had my 6 year old help me one time, he was so slow (careful) painting on the filler, by the time he was done with one side, the fin was pretty warped. However, when I do it quickly to both sides, I haven't had a problem.

You can also apply the F&F with a clean spackle blade. You don't need to water it down nearly as much this way. Use two blades to work a bead of F&F along the edge of one blade and then just drag it along the fin with the grain.

Green Jello said:

I think this is what I'm going to go with. The primer I used last time wasn't fill primer, so maybe that will help. I have two questions though:

1. In that list of steps, where do you glue them to the body tube? After the second coat of filler but before the primer?

2. How do you avoid the watered down wood filler from warping the fins? I've just been working fast to get it on both sides quickly and then smashing them under heavy books overnight.

Click to expand...

1. I completely build the rocket before I start on finishing. I regard the fillets as part of the finishing process, so I want them done before I try fixing any imperfections. The only time I've done any filling prior to final assembly is when it would be very hard to get to locations to sand. The Estes Alien Invader, and QModeling's upscale Mars Snooper come to mind.

2. If the fins are glued on before you start with the filler, you shouldn't have a problem with warpage unless the fins have a large span or are really thin. It's never been a problem for me, even with the few models that I've filled prior to final assembly. If your fins are thin, you might want to consider papering the fins prior to finishing to add extra strength.

JELL, I gotta go along with LW on this one ! Your sorta opening a can of worms. Best bet--as you build more rockets is to try everything and find the technique that suites you. That said , I use a bit of everything in my builds --but I use a LOT- of lumber in them. Most of my stuff has big fin area, so anything I do is amped up a bit. If I do not want to warp wood--here is WHAT I DO. I spray a coat of laquer based sanding sealer--lightly on both sides of the fin---it drys quickly--I do this a couple of times--let it set overnight and the put a good coat on , you can sand afterwards----clean up your mounting area --mount and then lay your joint--prime afterwards and sand as needed---one thing about getting rid of the grain is to sand off most of what you put on --so if your in a rush , my way won't work---but what the heck half the fun is the build and showing it off-- that leaves 50% for flying and more owws and ahhs

I've used drywall plaster with some success. After it's dry, it weighs almost nothing, and less than that after sanding. Fairly easy to work with, and it accepts paint easily.

I never tried drywall plaster. Is it plaster or sparkle. Plaster is very intolerant of moisture.

cwbullet said:

I never tried drywall plaster. Is it plaster or sparkle. Plaster is very intolerant of moisture.

Click to expand...

I checked the tub. It's plaster.

Seriously, it works. Just use a good primer first.

+1 on the CA method. Works great... Better than you'd expect.

GDJ said:

I checked the tub. It's plaster.

Seriously, it works. Just use a good primer first.

Click to expand...

I will tke you word and try it. I would assume clear coat would seal it anyway.

cwbullet said:

I will tke you word and try it. I would assume clear coat would seal it anyway.

Click to expand...

Try on a scrap piece first........just in case.
I'm 100% confident in the plaster method, but fate has a way of biting my @$$ now and again.

FWIW, I use basically the same process as SSenesy. Except I use Bondo spot putty instead of the undiluted filler to fix the imperfections. Who knows, a year from now I could be something completely different.

I must say GJ, with all the accolades you got on the finish on your Crossbow SST, I'm surprised you are not pleased with your current process.

Most spackle (drywall mud) is basically gypsum in a PVA binder. It's cheap and it works, but fill-n-finish does a better job. Fill-and-finish is more expensive, but since you need so little, the additional cost is practically meaningless.

LW Bercini said:

FWIW, I use basically the same process as SSenesy. Except I use Bondo spot putty instead of the undiluted filler to fix the imperfections. Who knows, a year from now I could be something completely different.

I must say GJ, with all the accolades you got on the finish on your Crossbow SST, I'm surprised you are not pleased with your current process.

Click to expand...

LW;

I'm with you on all my HPR and larger MPR models. Bondo spot putty works wonders. I use it over colloidal silica fillets to smooth them out. It beats sanding epoxy any day of the week.

Green Jello said:

1. In that list of steps, where do you glue them to the body tube? After the second coat of filler but before the primer?

Click to expand...

It may be a statement of the obvious, but any of the balsa filler/sealers used on the root region will act to fill in the balsa wood grain. If you apply the filler/sealers first, then whatever adhesive you use to attach the fin to the body tube will have a more difficult time soaking into the balsa side of the joint and will result in a weaker fin attachment.

Maybe your fin will be just fine anyway, as in a fin with lots of root chord length for an ample structural joint. Or maybe the pre-filled wood grain will cause just enough reduced strength that your fin happens to be the unlucky one that breaks off in flight. Your guess is as good as mine.

I personally like to get the fins sanded to shape first, glued into place second, then worry about surface finish later. If you do a little surface prep on a standard cardboard body tube (covered with a thin layer of plasticine, I'm not sure why) by lightly scuff-sanding the area of the BT that will be under the fins, and if you thin your water-based glue a little, and if you use the double-glue method, you will usually end up with some rock solid structural joints.

BTW, another option for balsa surface finishing is to brush on some Minwax wood hardener first (let dry/cure), then start applying fillers. The Minwax works the same way as a superglue treatment to seal the balsa so the fillers don't soak in as deeply (and are faster and lighter to apply, sand, and finish).

Just to add flame to the fire another thing to consider is either to paper the fins nd therefore avoid sanding entirely or use MonoKote.

Or do what i do and build helis and gliders flying au naturale

I have discovered I lack the one key ingredient to a perfect balsa finish

Wait for it


Wait for it

Wait for it

Wait for it





The key ingredient is patience

Green Jello said:

Since I got back into rockets, I've been doing the fins by painting them with watered down wood putty and then sanding. It works pretty well, but I can't seem to get them to look 100% like plastic. I've seen that some of you use sanding sealer. Which is the better method? Is there a third way? For purposes of this question, I'm talking specifically about balsa fins.

Thanks

Click to expand...

Paper the fins... first choice.

CA hardening followed by thinned carpenters wood filler if needed... second choice.

Maybe give Bondo Spot Putty (available at the auto parts stores) a try... it's basically the solids from primer paint with some lacquer thinner carrier...

Later! OL JR

SSenesy said:

LW;

I'm with you on all my HPR and larger MPR models. Bondo spot putty works wonders. I use it over colloidal silica fillets to smooth them out. It beats sanding epoxy any day of the week.

Click to expand...

I'm using it on my LPR birds - post primer. I do an initial sanding of the dried putty with 400 grit, then follow with an 800 grit wet sanding. It easily takes the putty down to filling only the imperfection and not sitting on top of the primer.

Another technique I'm using these days is to have 2 different jars of watered down wood filler each stained with a different food coloring. (side note: I've switched from Elmer's brand to Famowood because I prefer the way it brushes on.) Here is a picture of a recent project where I have already sanded down the first filler coat (red) and am applying the second coat (green). By having different colors, it helps me see where I have not yet sanded.

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.

A related tip with the Bondo putty: I had some major damage to the fines of my SHR Mercury Redstone. On a lark, I globbed on a goodly amount of the putty and let it dry. I knew it would be very weak so I toughened it up by soaking the blob with thin CA. I was then able to file/sand things back towards the profile I sought. I had to do several iterations and the unpainted but sanded result looked like red polished sandstone. primer and paint made it look good as new. It is heavier than the original styrene but that was not a concern on this rocket.

I have always used Aero Gloss sanding sealer on fins and Elmers filler on spirals. I have a Big Daddy kit that I have been slowly working on that needed some extra strength in the fins in order to throw some 3 and 6 grain motors at it so I gave papering a try using 110lb card stock. I am papering them post-build so it makes it a little harder to do but so far I am pretty impressed with the results. I used PVA wood glue for the lamination and then medium CA on each edge. I may need to go back and use a very small amount of filler to clean the edges up but I am very happy with how it is turning out.

I originally learned about the painting with watered down carpenters putty from a Tim Van Milligan video over on Apogee. He said to mix it to the consistency "a little bit thicker than tooth paste". After doing it on 3 rockets now, I think it's far better to go more water than that and make it like a thick paint. What do you guys do?

https://www.apogeerockets.com/downloads/Video/Building_Avion_2.mov

Green Jello said:

I originally learned about the painting with watered down carpenters putty from a Tim Van Milligan video over on Apogee. He said to mix it to the consistency "a little bit thicker than tooth paste". After doing it on 3 rockets now, I think it's far better to go more water than that and make it like a thick paint. What do you guys do?

https://www.apogeerockets.com/downloads/Video/Building_Avion_2.mov

Click to expand...

I've found the best consistency for me is to mix it until it's about the same consistency at regular hot dog mustard... just exactly like mustard... it brushes on easy and pretty thin, which minimizes sanding, and still fills very well.

I've never had any luck trying to fill tube spirals with CWF... I can get it on the tube fine, but when I'm finished sanding it off, the spiral looks empty like I didn't put a thing on it at all... guess it flakes out or something... best stuff I've found for tube spirals is the Bondo Glazing and Spot Putty at the auto parts store... for about $4, I got a gigantic toothpaste tube of the stuff (about a foot long and about 3 inches in diameter-- enough for a lifetime! (Well, it was only 50 cents more than a "mini-tube" about the size of a body filler hardener creme tube (plastic mini-toothpaste tube). The stuff is basically the solids from paint primer suspended in enough lacquer thinner to make it a paste. I just daub it into the spiral with my fingertip... you can use an old gas card (they're free for the taking off the top of the pumps at Walmart) to "squeegee" the stuff off the surface of the tube and into the spiral, or just daub it on carefully and wipe off any excess with a paper towel. A little sanding and the spiral is GONE. Dries fast too, and seems to adhere really well... I've never had the stuff turn loose... and best of all, it works in one easy step... Works like a champ!

Later! OL JR

Awesome. Thanks luke. I'm gonna pick up a tube of that today. My last project I filled the spirals with CWF and they were still there when I painted. Lame.

Green Jello said:

Awesome. Thanks luke. I'm gonna pick up a tube of that today. My last project I filled the spirals with CWF and they were still there when I painted. Lame.

Click to expand...

I think you'll like it. Just be careful and don't get it on your clothes-- it'll ruin them. Use paper towel to wipe it off your finger and then FAST ORANGE or some other pumice cleaner to get the remaining stuff off your finger. It dries rather quickly, so you have to work fairly quickly as well.

Otherwise it works great... just be advised that it smells fairly strong. Also, store your tube CAP DOWN so that the next time you use it, you'll get PASTE and not RED LIQUID, which tends to separate when it's left stored a long time. The red liquid REALLY stains, and it's useless for filling purposes.

Later! OL JR