Like Greynomad and Jacki, I would have thought
signboard was a fairly common word, and
adsorbing quite uncommon.
Wiktionary identifies
signboard as a US word, but other dictionaries, British and American, have no regional usage note. Trying to get a handle on international usage, I found to my surprise that the word is used much more often in English-language publications in Asian countries than in any of the "core" English speaking territories:
Regarding jem's comment that no synonym comes to mind, just after this topic was posted I was reading a novel (
The Coffin Trail, by Martin Edwards) and saw this sentence on the first page of Chapter 1: "A mile further on stood a wooden sign with worn lettering." Then on the next page I saw: "A board freshly painted in a blinding shade of yellow bragged that Tarn Cottage 'presented outstanding potential for sensitive refurbishment'." So it seems that in some contexts either
sign or
board can be used with the same meaning as
signboard.
Bearing in mind the comments of those forumites who hadn't heard of the word, and its relative rarity of use in many locations, I'll make it rare for the future.
As for
adsorbing, it's quite a technical term, from
adsorb, meaning according to the Collins dictionary:
to undergo or cause to undergo a process in which a substance, usually a gas, accumulates on the surface of a solid forming a thin film, often only one molecule thick
I'm convinced it's correctly classified as rare.