What product would you reccomend for previously painted decking( dark grey) preferably and non slip.

If you have a specific shade in mind for your decking we would recommend using Valtti Plus Wood Oil, which is available in a wide range of colours. 

Valtti Plus Wood Oil is a water-based natural wood oil provides weather protection for new and previously oiled wooden furniture and decking. Click here to shop. If you prevent a solvent based solution, then there is also Valtti Wood Oil (available here) 

Before application, remove loose material with a scraper and/or a wire brush first. Clean the surface and, if necessary, wash with Homeenpoisto or Tehopesu according to instructions.