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Here is a brief tutorial for those who are as sloppy and careless as myself.

Bit of a nasty ding and this is after steaming multiple times. . . . .



No idea how it happened, luckily this appeared early on in a build. 99.999% of the time they decide to make an appearance just as one is doing the final polishing.
Not to worry. Providing we have a suitable piece of Spruce we can make it all but vanish. An off cut from the soundboard is the obvious choice. If we choose the piece from the same side of the soundboard we won't have a problem with colour/reflection mismatch. It is much more difficult to execute a near invisible repair if we have to select from pieces of Spruce that do not belong to the original soundboard.
The diagram shows the shape of the patch needed - the so called rugby ball (or American football) shape. The dotted line within the patch indicates the ideal of matching a grain line - fussy but helps to fool the eye into believing we are looking at original wood.



I use a Scalpel to carefully cut out the shape on the original soundboard.



Now we need to cut a similar shape on our off-cut, making sure that it is a little larger than the cut on the original soundboard.



Now it is a simple matter of transferring our patch onto the crevice we created on the soundboard. Don't flip the piece and create reflection problems. I glue with Hide and gently compress the patch into place. Clamping helps.



Allow to dry and then scrape or sand the patch level.



In pragmatic terms it is 'invisible'. At least it's good enough for my ageing eyes. Picking out a grain line went a long way to the success of this particular repair. I had 3 goes at this to match the grain. Sometimes it can take many more attempts than that.
Just remember: Practice on scrap.

Michael - thanks for posting, that's very impressive. I'm off to the scrap pile to practice.

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Dave White
De Faoite Stringed Instruments
". . . the one thing a machine just can't do is give you character and personalities and sometimes that comes with flaws, but it always comes with humanity" Monty Don talking about hand weaving, "Mastercrafts", Weaving, BBC March 2010

Well done.

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"Act your age, not your shoe size" - Prince