Remove the painting from its hanging, and carefully lay it on a flat, clean surface. Wear the plastic gloves to protect you hands when handling cleaning chemicals.
Apply some neutralizer to the piece of cotton and gently wipe it over the surface of the painting. This will remove any surface debris.
Establish which product will clean the painting best. Apply a small amount of emulsion cleaner and varnish remover to different swabs. Gently roll each over a corner of the painting and judge which removes the most grime. Emulsion cleaner removers dirt and smoke better, while varnish remover will remove faded, warped or stained varnish. Immediately after each test apply a small amount of neutralizer over the area, using the cotton, to stop the chemicals reacting.
Clean the painting with the appropriate chemical. Dip a fresh cotton swab into the solution and gently roll the swab over the painting. Do not re-dip the swab once it has been used. Simply use another once it becomes dirty. Clean a small area at a time, applying neutralizer after each application to protect the painting.
Varnish the whole painting again once it has been cleaned. Varnish can be bought in a spray or brush on. Consult the packaging on how best to apply the varnish.