Filling Tube Spirals - Revisited

Back in Part 4 of my Skill Level 1 series, Building the Big Bertha, I talked about filling in those spiral grooves you find on your rocket body tubes.

My first build - Der Red Max. I hadn't yet learned that you can fill in those spiral grooves.

This makes the rocket look better (it doesn't look like you made it from a roll of toilet paper), and might even make it fly better.

In any case, I like my rockets to look nice close up, because they spend a lot more time on the ground than in the air, and I like to show off my work to people who come to visit.

Some people don't bother to fill in the spiral grooves, because you don't see them while the rocket is flying. But if you're fussy about rockets like me, you try your best (I usually make a few mistakes) to get all the details.

Since building the Bertha, I've refined my technique, and I want to share it here. I've now gotten it to where I can fill in those spirals quite effectively - and this technique requires almost no sanding. That's particularly important for me, because I've never managed to sand wood filler off a body tube without going through the smooth surface and raising a few paper fibers - which show up in the final paint job.

Here's how I now fill tube spirals, and I hope you find it helpful.

First, you need your old friend, Elmer's Carpenter's Wood Filler - the kind with the orange lid, which looks like this:

Get only this stuff for rocketry purposes. Don't get the other stuff - it's grainy and weird, and doesn't sand well.

As is often the case, I pick up a lot of my building tips from Chris Michielssen's blog, and that's where I got the razor blade idea, which I still use, but with a twist - plastic razor blades.

I didn't even know they made these until I saw them at a hardware store. I grabbed some, and thought I'd try them, and if they worked, I'd report here that now even kids can safely fill tube spirals using the razor blade technique. It turns out, these are even more effective for me than metal blades.

Chris uses a dull hobby knife blade to lay some CWF into the spiral groove (see here) with beautiful results. I haven't quite got the hang of that technique. Back when I was building the Bertha, I used a small paint brush to fill in the spiral with thinned CWF.

This technique was OK, but I had to overthin the filler a bit, and it wouldn't go evenly into the groove. There would be a lot of low spots, and I'd have to re-fill the grooves after priming and sanding the rocket, and it didn't always turn out great.

Now I use a medicine dosage syringe.

Pharmacies sell these mostly for measuring out cough syrups and other liquid medicines for children. They work great for getting wood filler into the grooves.

A good first step (again, a Chris Michielssen tip) is to use a sharp pencil to trace along the spiral groove.

Though not strictly necessary, it will help you see the groove clearly as you apply filler. Some grooves are deep and obvious; others are thin and very light. I pencil in all my grooves, just to be sure to have a guide line.

Next, you'll thin - slightly - some CWF. This shouldn't be as thin as you'll need it to paint onto the fins for filling. It just needs to be thin enough that you can draw it up into the syringe. I spoon some out into a small bowl, then add water bit by bit with an eye dropper. I stir until it's nice and smooth, no lumps. Continue adding small amounts of water and stirring until it's about the consistency of cake batter - or perhaps a little thicker, say, between cake batter and peanut butter.

Stick in the tip of your syringe and draw an inch or so of CWF into the syringe. Wipe off the tip of the syringe.

Use the syringe to dispense a line of CWF right into the spiral groove, about an inch or two at a time.

I should have worked in the other direction, so you could see the filler going into the groove. But it doesn't matter.

Next, you'll pick up one of your plastic razor blades.

A regular razor blade will work for this - essentially, you're using it as a little putty knife. But the plastic blade has a couple advantages, I find. First, there's no risk of cutting yourself - or the rocket. With a metal razor, I sometimes cut or scrape into the body tube a little with a corner, raising paper fibers - that won't happen with a plastic blade. Second, because the blade is plastic, you can apply downward pressure. So instead of just scraping off the excess filler, you can actually push the filler into the groove as you go.

The blade has a beveled face and a straight face. Leading with the beveled face, you apply pressure, and the edge of the blade flexes, flattening the filler and completely filling the groove.

Both edges of the blade have a flat face and a beveled face.

So with the blade, follow the groove and remove the excess filler.

See the edge of the filler? That will have to be sanded off later - unless you do this next trick.

What you're trying to do is two things: completely fill the spiral groove and minimize the sanding you'll have to do once the filler is try. This part is especially true for me, because I've never been able to sand filler off a body tube with ordinary sand paper and not mess up the surface. Apparently I'm alone in this, because I've never heard of anyone else having this problem. But in any case, sanding is a pain.

So I came up with this last little trick - the part of the process that makes it all even more effective, almost eliminating the need to sand when you're done. Using the pad of your finger, buff the filler you've just applied. This will take off any excess and smooth out the edges of filler left by the razor blade. Just buff lightly - across the groove, not with it.

You'll know you're removing excess filler because after a few times, a small amount of it will cake on your fingertip. You'll need to wipe or wash that off occasionally.

Let the filler dry completely, then check it to see if you can feel any bits of the groove that were not filled in, or any raised ridges you didn't buff off with your finger. You may find that you don't need to sand at all!

I always do sand, just for good measure, but now I only have to make a cursory pass over the groove, and sometimes I find a glob I accidentally got somewhere in the middle I have to sand off. Because regular sandpaper messes up my body tubes 100% of the time, I now use a sanding sponge - the finest grit I can find, which is 320.

The sanding sponge is soft and flexible, and conforms to the body tube, and it won't cut into the smooth surface.

That's it. You'll want to wipe off any dust before you prime and paint, but everything should be filled in perfectly, and the surface will be smooth and groove-free. If you notice any low spots you missed in the groove after you put on primer and sand it down, you can use a bit more CWF to fill those in - but I find with this technique I almost never have to do that.

I hope somebody tries this technique and likes it. I used to get so frustrated filling tube spirals, because I could never quite get them filled, and because when I sanded them down, I'd have scratches and bumps that looked terrible once the rocket was painted. Now this is a part of rocket building I enjoy.

Now if only I could find a way of filling wood grain on fins I looked forward to!

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