The first step in cleaning an oil painting is removing the painting from its frame. Make sure the frame is on a soft surface to avoid damage. Once the frame is off use a vacuum to clean the crevasses where the painting meets the wooden stretcher. Take care not to chip the paint.
The next step is removing mildew. Using a cotton ball and a Sodium Hydrochloride cleaner, gently rub the affected spot on the painting. Before continuing, check the cotton ball to ensure the paint is not coming off. Once the area is cleaned, immediately wipe the cleaned area with a wet sponge. Make sure to get all of the cleaning agent off.
To clean off a small amount of dirt and grime, use an onion cut in half. The potency can be increased by adding drops of lemon. Do not allow the onion lemon mixture to dry on the painting. Wipe off with a wet sponge. If the dirt is still holding on, move up to cleaning with a lemon detergent. A soft sponge should be used with warm water. Use as little water as possible. After cleaning, wipe off all detergent.
For a higher level of dirt and grime use Sodium Carbonate. The concentration will need to be determined before using the solution on the entire canvas. The way to do this is dissolve a small amount of Sodium Carbonate in warm water, then try the mixture on a corner of the paining with a cotton ball. Continue this process until the mixture takes off the grime without taking off the paint. After the cleaning, rinse the Sodium Carbonate off with fresh water or it will leave white streaks.
If the painting is so dirty the above methods don't work, the next step is to remove the varnish. What is needed to remove the varnish is methylated spirits, turpentine diluted with linseed oil, and several cotton balls. Use the cotton ball covered in methylated spirits to remove the varnish. It is important not to go to deep, remove only the varnish and not the paint underneath. The method used to protect the paint underneath is after the varnish is removed, use a fresh cotton ball covered in the turpentine-linseed oil mixture to stop the methylated spirits. Continuously check the cotton ball to ensure the paint is not coming off. If paint starts to come off use the turpentine linseed oil mixture to stop it.
After cleaning the paining it is necessary to re-varnish the painting. The varnish can be gloss or matte and should be applied in a crisscross pattern with a varnish brush. Allow to dry.