Hi, all depends on the condition of the seal at present , if the seal is broken away in traffic areas, white or cloudy or stained in areas- then you can test strip with wax off- or your chosen stripping agent. Leave 6-7 days or until fully dry - re-seal with the slate seal, or your chosen finish.
If the finish is in resonable/good condition and stain free , just needs a light de- grease- test two tiles with diluted power clean- ( water based alkali)1-15. Work in very small sections, application for only 1-2 mins as heavey cleaners can whiten the remaining seal if left on to long,( you will need to strip if this occurs on test) degrease with product and white pad , rinse and towel dry each section, leave again for 4-5 days or until fully dry and then roller ( small foam raidiator roller i find best for this) a light top up coat on and perhaps a second if required after first is dry.hope this helps a bit.
