The phrase "a layer of sweat" is correct and usable in written English.
It can be used to describe a physical condition where sweat accumulates on the skin, often in contexts related to exercise, heat, or stress.
Example: "After running for an hour, I could feel a layer of sweat covering my body, making my clothes cling to my skin."
Alternatives: "a film of sweat" or "a coating of sweat".
Rather than a monkish waterproof cassock, this Rambo is seen running around the battlefield in little more than a layer of sweat.
Sally Abe turns up to her OFM photoshoot on a motorbike, straight from an "insanely busy" lunch service on the hottest day of the year, but even under a layer of sweat and kitchen grime, she still seems fresh.
Bathed in a layer of sweat after going through a series of shooting and dribbling drills, Rose skied above the rim and dunked, eliciting a cheer from the handful of fans who had arrived early.
She gives him one of those pelvic grinds of hers, drawing closer to him than a layer of sweat, and he remains unmoved, unlike the phalanx of saps she has driven to distraction in those semi-noirs best relegated to 2 a.m. runs on Cinemax.
It depends on if I'm adding in a layer of sweat from a workout or not.
We've finally reached that special time of year when the weather is perfect and crisp and you remember that people in your city can actually be good-looking when they aren't drenched in a layer of sweat and ball sack smell.
Hunched over the handlebars of a stationary bicycle, Sarah Reinertsen vigorously pumps the pedals as a thin layer of sweat coats her back and shoulders.
After a half-hour of work, Reyes bounded into the clubhouse, a healthy layer of sweat accompanying the wide smile on his face.
A thin layer of sweat glistened on her forehead.
A fine layer of sweat covers her oversized forehead.
Tay-lor!," Swift appeared, in a baggy pink striped sweater, her hair in loose braids and her skin showing a faint layer of sweat.
