The short answer
Find the render system or manufacturer details, photograph the affected wall and defects, then ask a provider to propose a method consistent with that system's maintenance instructions.
Recover the product information
Look for installer paperwork, a warranty or product records from the building work. A white wall is not enough to identify the render system. If the finish remains unknown, say so in your enquiry. The provider needs to establish what they are treating before the scope can be settled.
Describe the marks
Send a wide photograph of the elevation and a closer image of the affected area. Mention whether the marks are below a fixing, beneath an outlet or across a broad part of the wall. Flag cracks or areas that sound or look loose if already known, without disturbing them. Different problems can look similar in a photograph.
Use manufacturer guidance
K Rend warns that very high pressure can damage its render and discusses maintenance approaches for its products. That guidance should not be applied indiscriminately to every other render system. Ask the provider to identify the relevant instructions for your finish and explain how the proposed work follows them.
Agree a trial and protection
Ask whether an inconspicuous trial area is appropriate, how the result will be assessed and what happens if the finish responds differently from expected. Include windows, vents, planting and adjacent surfaces in the protection plan. Ask about any treatment precautions and when the area can be used again.
Separate cleaning from repair
A cleaner surface will not resolve a structural crack or detached finish. If defects are identified, ask for the appropriate assessment and an amended scope. Keep cleaning, repair and repainting or coating decisions distinct in the quote.
Sources & further reading
Sources checked 2026-09-13. Statistics retain their original reference year.