Monday, September 6, 2010

Carpenter's Wood Filler Mix Ratios

For simplicity's sake, I'll refer to Elmer's Carpenter's Wood Filler as "CWF".

Years back, I used sanding sealer to fill balsa wood because that's what we were told to do in the kit instructions. Many old school builders still use it and swear by it.

I now use (and swear by) Elmer's CWF. If you read the forums, some still call it by the old name of Fill N' Finish or FNF. Elmer's no longer makes it under the Fill N' Finish name. Essentially it still does the same thing - it easily fills the balsa pores.
EDIT: Buy the CWF with the orange lid. Stay away from the stuff labeled "MAX". It is impossible to sand!

I use the CWF for good reasons:
1. CWF doesn't smell as bad as sanding sealer.
2. I can apply CWF indoors.
3. It's a lot cheaper than sanding sealer.
4. CWF won't load up your sandpaper. The sandpaper lasts many times longer.

Some complain about how much dust is left after sanding with CWF. I sand a lot of bare balsa, there is dust everywhere anyway.

In the past I've just scooped out some CWF into a small Tupperware style container and added water. I'd go back and forth adding a little CWF or water until I thought I had the right mix.

For this blog, I thought I'd try to find the correct ratio of CWF and water for a good, brush-able mix.

The mix of CWF and water was not what I expected.
I used measuring spoons a baker would use to add dry ingredients in a mix.

I started with one level tablespoon of CWF.
I added one level tablespoon of water. It was stirred to mix. The lid was snapped on and shook to mix a little more.
It was very thin. If brushed on thin balsa, if would warp the wood.
Another level tablespoon of CWF was added. After mixing, still a little thin. More CWF was still needed.

In the end, here's a workable mix ratio:

1 Part Water
2 1/2 Parts CWF

I can hear the reader now: "That's not the way I do it!"

Of course it's not - everybody has different technique that works for them. But, this could give a first timer a good place to start.


  1. I built a rocket with my son over the Thanksgiving holiday. I used CWF and my highly calibrated eyeball (translation: I felt like a mad scientist) to add water to thin it out in order to fill the seams. I didn't think to use it on the balsa wood nose cone. I'll keep this tip in mind for our next rocket. Indeed, I am a beginner, and this was a great place to start. Thank you!

  2. Hi Beowolf,
    I'm glad the tips helped. By all means, use it on the balsa nose cone. Brush it INTO the balsa grain pores, let dry and sand off. Nose cones always fill easier because you are dealing with balsa "holes" as opposed to long balsa grain lines on the fins. Have fun with your son and the rockets!

  3. Chris,

    Your mixture ratio worked out great! I ended up adding a splash more water, because I suspect my tub of CWF is older, and drier. I noticed that the final mixture is a little "grainy;" is that normal?

    I brushed it into the pores of a balsa wood nose cone as you advised, and then sanded to a gloss finish with 600 grit sandpaper. The end result looks wonderful! I also applied a thin layer to the fins, but the long grain lines to which you made reference still persisted. How do you get rid of those?

    Thanks again for your suggestions!
    Todd / Beo

  4. Regarding the long balsa grain lines in the fins:
    Some filler can work it's way out of the long balsa grain pores when sanding. Usually any remaining filling is taken care of using primer.
    So: Filler and sand, then primer spray and sand.
    That should fill the remaining lines.
    The filler stays in the nose cone pores because they are a deeper "hole" rather than a shallow "trough" in the fins.
    Hope that helps!

  5. Whoops! - sorry, didn't see the first question about the filler being "grainy".
    The filler can be a little grainy, that's normal. Just be sure you mix it up really well before brushing it on the balsa.

  6. Should I apply CWF to the fins and body tube before gluing the fins on? I'm currently working on a rocket with through-the-body fins, and I'm not sure if it will interfere with the glue.

    Daniel of Bloomington

  7. You can, for me it's easier to Fill the wood grain with the fins off the rocket.
    CWF is water based and a water based glue will get into the wood with the CWF filler on. I remove almost all of the CWF filler.

    1. Oh, help!

      I applied the CWF as you directed, and as I was sanding it off, I was so impressed by how smooth it was.

      But as I was trying to sand it all off, I may have gone too far. I got to a point where it looked like I was actually scuffing up the body tube! I'm not sure if this is a good or bad thing.

      I think I read on some Apogee Rockets newsletter that you want to rough up the tube before applying a couple coats of primer, and then wet sand that, but I'm not sure about that.

      I applied a new coat of CWF that I can sand down, and I await your advice. Thanks!

      Daniel of Bloomington

  8. Hi Daniel,
    You really want to sand "to surface".
    When sanding CWF, use 400 grit. Anything rougher than that and you will scuff the tube. Even though 220 is called a fine grit, it's too harsh for tubes and balsa.
    The last tube I just filled was roughed up more than I would have liked.
    After a spray of primer/filler and sanding, all the roughness and fuzzies were gone.
    Simply spray with primer filler and sand (lightly) with 400 grit. It'll come out smooth.

    1. I think I was too impatient with my first sanding. I recoated and re-sanded with 400 grit paper, and apart from a few blemishes, I think I'm good to go.

      Sorry to keep asking so many questions, but I find your blog so useful!

      One last one - do you wet sand your primed rockets, or dry sand? I've seen both techniques referred to online.

      Daniel of Bloomington

    2. I usually dry sand the primers. It's too close to the bare tube and bare wood on the fins and nose cone.
      If water gets in the wood or tube fibers they will swell up.
      I don't wet sand until after the color coats go on. By then you have less chance of sanding down into the fins or body tube.