Crossword puzzle clue "coatings of frost"

HOAR, to make hoary, grey with frost or age

sceptic, denying the possibility of knowledge

acting to stop a car window from fogging up

intended to prevent rust

ARIL, an additional envelope, often fleshy, developed around the seed in certain plants (e.g. yew), also ARILLUS

ASH, to convert something into ash

DEW, to wet with condensed moisture, also DEAW