In current usage, only standard for forming nouns (e.g.,cheerfull is now a misspelling).
For nouns, both forms have historically been considered acceptable; e.g.,Weld’s Progressive English Grammar provides bothspoonful andbowl-full as examples.[1] Now usually hyphenated (e.g.,bowl-full instead ofbowlfull), though both the spelling-ful and spaced compounds withfull are more common (e.g.,bowl full); however, the plural is rarely formed by suffixing-s instead of pluralizing the noun when spaced (e.g.,bowl fulls versusbowls full), unlike with-full (bowl-fulls/bowlfulls orbowls-full/bowlsfull) and may be considered nonstandard.
^Allen H[ayden] Weld (1859), “The Noun”, in George Payn Quackenbos, editor,Weld’s Progressive English Grammar, Illustrated with Copious Exercises in Analysis, Parsing, and Composition, Adapted to Schools and Academies of Every Grade, Portland, Me.:[…] O. L. Sanborn & Company; Boston, Mass.: Chase & Nichols; Philadelphia, Pa.:J[oshua] B[allinger] Lippincott & Co, part II (Words),page44: “Nouns ending inful andfull, and compounds in which the principal word stands last, annexs ores, to form their plural; as, spoonful,spoonfuls; bowl-full,bowl-fulls; man-trap,man-traps; step-son,step-sons.”
