。
The boiler got coated thick with limescale.
Literal
Boiler [topic-は (dropped)] kettle-scale [subj-が] densely stuck.
The topic boiler has no explicit particle — は is dropped, common in headline/announcement style and casual speech. 湯あか is the Japanese word for 'limescale / kettle fur' — the white calcium deposits that accumulate from hard water in kettles, boilers, and bathtubs. びっしり is a state mimetic for 'densely, thickly packed,' typically of small things filling a space. つく 'to be attached to' is the stock verb for things sticking on surfaces.