A Email Question About Filling BT Seams

Chris-

You frequently explain how to fill the spiral seam on a rocket tube. There is a detail, however, which is not clear; I thought it might be something of general interest to your blog - reading community.

There are typically two spirals on a body tube. There is a "primary" spiral, where the gap is open to the air, and a harder-to-see "secondary" spiral, which seems to be under the outer layer of paper. 
I have seen on your blog and in the article you wrote for the Apogee newsletter that you use a pencil to highlight the secondary spiral. But what is not clear is how you fill this secondary spiral. 
Do you somehow rupture or slice through the paper covering it so that you can push the filler down into the groove, or do you simply put filler on top of the outer paper layer? (Clearly, for the *primary* spiral, you can push the filler down into the groove.)

If I knew the answer to this question, I could do a much better job of filling spirals!

O.S.

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Hi O.S.,

I only fill what you refer to as the "Primary Spiral" where the gap is open to the air. 
You can feel it with your fingernail.
The Secondary spiral is never filled with wood filler.

I wouldn't want to punch or cut into through the clear glassine skin, 

that would weaken the tube.

Usually any slight secondary spiral indent is filled with the sanded filler/primer.

That pencil line is only in the primary, outside spiral. 

The line makes the spiral easier to see when filling with the CWF.

Thanks and good luck with your BT seam fills - 

Chris Michielssen 

www.oddlrockets.blogspot.com www.modelrocketbuilding.blogspot.com