The short answer
Photograph the marks and identify the paving before choosing a treatment. Black spots are a description of appearance, so a provider should assess the cause, surface and likely result rather than promise that one wash removes everything.
Show the pattern, not just one spot
Send a close-up and a wider view showing where the marks occur. Explain whether they appeared suddenly, have spread or remain after ordinary cleaning. Mention previous products and any recent building or garden work. These details help the provider decide what needs assessment; a photograph alone should not be treated as a definitive diagnosis.
Identify stone, joints and coatings
Find purchase or installer information if possible. Include the joints and any cracked, flaking or coated areas in your photographs. Different paving products have different aftercare instructions, and a product suitable for one material should not be assumed suitable for another. Marshalls’ aftercare resources illustrate why the product and type of problem matter when deciding how to clean paving.
Agree the limits of a trial
If a test area is proposed, ask where it will be, what will be applied and how the result will be assessed. Confirm whether the test is included in the quote and what happens if it does not produce the desired finish. Ask about appearance after drying, possible remaining marks and any follow-up. Do not use an immediate wet-surface photograph as the only measure of the agreed outcome.
Include protection and aftercare
Show plants, drains, outdoor furniture and adjoining surfaces. Ask the provider how those will be protected, how washwater will be managed and when people and pets can return. These arrangements should follow the proposed product and site conditions. Keep specialist stain treatment distinct from ordinary washing, joint work and sealing when comparing the totals.
Sources & further reading
Sources checked 2026-09-13. Statistics retain their original reference year.