It might be a good idea to bring someone in to do this- an industrial hygienist type person.

But COSHH starts with identifying the hazard:

• What is the dust you are looking at made of: check with EH40 to see if it has a WEL
• Try to assess if the WEL will be exceeded-you may need to do some monitoring
• Take into account how often people are exposed and how many people are exposed

Apply the hierarchy of controls.

Ideally try to prevent exposure altogether; ask:

• Do you need to use that particular substance? Can you replace it with something less hazardous
• Can you isolate the process in a cabinet or box
• Can you use a robot(you probably can’t but you ask the question)
• Can you reduce the number of people working in this area
• Can you use a local exhaust system to remove the dust
• Does the sander have a built in extraction system? What happens to the dust at the other end of the extraction system? How is it collected
• How do you clean up any dust that settles- don’t use a broom use a suitable vacuum cleaner
• Do the personnel have access to cleaning facilities? Can they remove their overalls without creating more exposure risk- could you use a disposable set of overalls
• PPE( including respiratory protection) is a last resort but if you go for make it suitable and make sure that it is properly maintained
• Does it fit the user- face fit testing for masks
• Make sure that they have somewhere to store the PPE  safely so it does not get contaminated

And the rest….

PS You don't COSHH assess a substance you assess the process the substance is involved in.

With respect, this is not strictly correct. Consider what is now in the current ACoP:

Paragraph 10 - Employers should regard a substance as hazardous to health if it is hazardous in the form in which it may occur in the work activity. A substance hazardous to health need not be just a chemical compound, it can also include mixtures of compounds, micro-organisms or natural materials, such as flour, stone or wood dust.

In fact, even the original COSHH regulation covered this point. In the list of definitions of what constitutes a hazardous substance we have the following:

“(e) which, not being a substance falling within sub-paragraphs (a) to (d), because of its chemical or toxicological properties and the way it is used or is present at the workplace creates a risk to health” - COSHH Regulation 2 (1) Interpretation 

My inerpretation of this is that you will need to establish what is in the paint and thus will be in the dust and base your risk assessment on this. This is just the same procedure as you would adopt if you were sanding down very old paintwork in a building that might contain lead. Treating this simply as nuisance dust is not really acceptable. This indicates an approach to the workplace that applied the original paint to obtain the relevant information about constituents and what might be the residual chemical once the paint has fully cured. 

Chris

If you are unable to determine actual constituents then you have to assume worst case so possibly Lead and its salts, other pigments (Iron salts/Cobalt salts/Titanium Dioxide etc.) along with the carrier matrix e.g. cellulose and if your sanding paper is quartz (sand/silicon dioxide) based the possibilty of RCS