Pityriasis rosea: This condition is characterized by the sudden onset of flat, sandpapery lesions, often on the back, and can appear overnight. The lesions typically start with a herald patch, followed by a secondary eruption of smaller patches.

Contact dermatitis: This is a type of inflammation of the skin that occurs when the skin comes into contact with an irritant or allergen, leading to red, itchy, and sometimes sandpapery lesions.
Eczema (atopic dermatitis): While eczema is more commonly associated with itchy, raised lesions, some forms can present with flat, sandpapery patches, especially in the acute phase.
Psoriasis: Although psoriasis typically presents with well-defined, scaly plaques, early or mild cases might appear as flat, sandpapery lesions, especially on the back.

Meningococcemia: This is a severe infection caused by Neisseria meningitidis, which can present with a petechial or purpuric rash that might initially appear as flat, sandpapery lesions. It's a medical emergency.
Septicemia: Bacterial septicemia can cause a variety of skin manifestations, including lesions that might resemble sandpapery patches, especially in the context of systemic illness.

Pityriasis lichenoides: A rare skin condition characterized by the appearance of small, scaling papules that can resemble sandpapery lesions. It can be chronic or acute.
Lichen planus: Although typically presenting with purplish, itchy, flat bumps, some variants of lichen planus can have a more sandpapery appearance, especially on the back.
Secondary syphilis: This can cause a wide range of skin manifestations, including a maculopapular rash that might appear as flat, sandpapery lesions on the back, among other areas.