Cover a table or workbench with soft cushioning material such as polyurethane foam. Place the painting face down on the covered work surface.
Vacuum any dust from the back of the painting and the frame. Use care when vacuuming the canvas.
Remove the nails holding the painting in the frame, using pliers. Lever the pliers against a metal ruler to avoid damaging the frame.
Clean between the canvas and the wooden stretcher, using the crevice tool on the vacuum cleaner. Vacuum gently to avoid cracking or flaking the paint.
Cut an onion in two. Rub half the onion gently over the painting. Cut small slices off the flat surface of the onion throughout the process to refresh it. Add a few squirts of lemon juice to the onion for a stronger cleaning effect. Wipe the painting with a damp, soft sponge and allow it to dry.
Mix warm water with delicate lemon detergent for deeper cleaning. Soak a soft sponge in the soapy water and wipe it over the painting. Be careful not to soak the canvas. Allow the detergent water to sit on the canvas for a few minutes to loosen the grime from the painting, then wipe off with clear water. This medium level of cleaning may be necessary if the painting has been in a smoky environment.
Dissolve a bit of sodium carbonate crystals in a cup of warm water. Dip a ball of absorbent cotton into the water mixture and test it on a small corner of the painting by rubbing it on with a circular motion. If you see little cleaning effect, add a bit more sodium carbonate to the water. Continue the process until you have achieved the proper solution to remove grime from the painting. Wipe the solution over the painting with a sponge. Then use a sponge dipped in clear water to remove the solution from the painting. Sodium carbonate can leave white streaks on the painting if overdone or not properly rinsed.