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I've never seen "sondage" in this kind of context --but I'm not an expert on resotration techniques.

In my experience, the term is used in archeology, refering to a "trial trench" which is done at the beginning of an excavation to determine the most basic stratigraphic layers of the site.

Typically its a relatively small --but deep-- hole (about three meters square, at the site I worked on), which extends all the way down to "virgin" soil (or to something solid, like a pavement).

The basic idea is to try and obtain the maximum amount of information about a site with a minimum of disturbance to that site.

Thirty years ago, if one wanted to excavate an archeological site in France, a "permit de sodage" was the first step necessary to obtain the more comprehensive "permit de fouilles".

Clearly,what is intended in your context is the "excavation" of a very small area of the piece, in order to determine the "stratigraphy" --the various layers-- of the paint, down to the original layer.

If there are several layers of pigment, these are reflective of the tastes of the various periods in the life of the piece.

For instance, American kitches of the late 19th century would probably have been finished with a simple clear varnish over the woodwork; in the 1920s or '30s (as the varnish darkened), they would have been painted a nice bright, sanitary White; in the '50s a bilious lime Green; in the '70s a hot Pink.

All of these layers would have been progressively (and briefly) visible in the '80s, when the new owners stripped off all the paint in order to get back to the "natural" Oak beneath, thus destroying the History of Taste recorded on the woodwork of the kitchen.