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You know like the chalk that some use to dust up dovetail joints so that the chalk marks the part of the wood that needs to be removed? I'm trying to fix this old girl up now and have seen this method done with chalk or even carbon paper. I'm not sure what kind of chalk. Is there a chalk specific to wood working? Can you just brush and vacuum it away when you are done? Do you put the chalk on the male or the female piece? So many questions. I've never used chalk for such repairs but in this case using a scarf joint chalk seems to make sense.

Regards.

I use regular chalk sticks for black board
the nice thing is it removes with a wipe

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

Sidewalk chalk is fairly soft and cleans up easily.

I use chalk a lot; like on the v neck joint on the little baroque I started working on again yesterday. I chalk up the part that I fitting, and scrape off the high points on the other part until it is mostly chalk marks. On the neck, I chalked the head, because the neck part is in the open, and it is far easier to keep the head straight than to try to keep the neck straight.
Fitting a patch on a sap pocket, similar to what you are doing, I would carve the curve in the patch material, with a hand hold still on it (or glue one on); chalk the patch up, and press it into the depression where the pocket was.
Chalk isn't always that easy to get off. Be sure to keep the area dry; don't get it wet. The dryer stuff is best. Like Barry says, sidewalk chalk is probably better than the dollar tree stuff I have! You can be too cheap.

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Why be normal?

You can use white "chalk line" chalk to dust on the pieces with a brush. The blue is harder to remove and the red is semi - permanent.

I just use chalk sticks for blackboards, buy in toy stores or though ebay..
Wipe with naptha if an eraser doesn't remove any traces.

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Pick one side of the joint to call 'fixed', and chalk that, removing wood only from the other side. On a V-joint, for example, I like to cut the point first, to make sure it's centered on the neck.That gets chalked, and the wood is removed from the mortise. This is backwards from normal woodworking practice: it's usually harder to get the mortise right, so you cut that first and then fit the tenon to that, but in this case the centering of the V is important.

In something like that top repair I'd try to get the top surface right, and then fit the patch to that. There are limits as to how much material you can remove from the top, whereas it's easy, and normal, to start out with a thick patch that then gets dressed down to level after it's been glued in.

I like sidewalk chalk myself. When everything is fitted up wipe off as much as you can with paper towels, and then get the rest with an soft rubber eraser.

Thanks all for the replies. Ken great idea on making a handle. I can use HHG to mount a little spruce handle and make like a rubber stamp and working from just the one side.