To make the decision to clean an oil painting will depend entirely on how grubby it is and how valuable it is. Taking advice from a professional is your best choice, but failing this there are some methods which can be used to clean your painting at home or in the workshop.
One thing that you must remember is that you should NEVER allow water to come into contact with your painting. Water soluble oils have been around for many years and by allowing water to come into contact with the surface, you may ruin a valuable painting in an instant. Soap or detergent and water can also seriously damage the old traditional oil paintings by inducing a kind of milky reaction to the oil paint.
If you dampen the canvas in any way it can also cause stretching which will lead to a bagginess to the picture. Water can seep into any imperfections or cracks within the paint and cause flaking.
Oil paint is a combination of linseed oil and other various pigments and the integrity of the painting could be seriously damaged by using any kind of thinners, alcohol, or abrasives (such as the scratchy pads used for washing dishes)
If your painting is generally a little grubby from hanging, then you may be best advised to lay it flat upon a work top and dust it with a feather duster to get rid of any loose fluff and dirt. You can also use a hoover with a soft attachment, but failing this, an elastic band and a piece of muslin over the end of your nozzle to avoid scratching the picture surface.
A good method to remove general grease and accumulated is dirt is to use a piece of bread. You may need to use a couple of loaves depending on the size of the painting.
Lightly brush the bread against the surface of the paint and you should see the dirt being absorbed by the bread. When the piece of bread gets grubby, then take another piece. This is a good method to use when there is light soiling ( it's useful for cleaning old books too) When you have finished, take a soft brush to flick out any leftover crumbs.
Other home remedies include using a cut potato or an onion, to act as an astringent with a couple of drops of lemon juice. Just keep gently wiping over the picture and throw away as each piece absorbs the grease and dirt.
For a more thorough job, the best method to use for cleaning an oil painting, is to use a lanolin based cleaner (such as an unscented or plain hand cleaner) Using a soft cloth or cotton bud take a little of the cleaner and with a circular motion gently dab the cleaner onto the painting. I would recommend starting in the very bottom right hand corner to test for fastness. To begin with take a small amount on a cotton bud and wipe on; leave for a couple of minutes and then dab gently off.
If the paint has had no reaction to the cleaner than it will be OK to finish the job. Take your time and never try to rush it. If you scrape too vigorously at the surface you will damage the painting and be in danger of causing the paint to flake.
Sometimes you may find that you cannot bring back the total freshness of the painting because the original varnish has yellowed over time. This means that you may decide to speak to a specialist about having your painting restored to its former glory or you may even feel confident enough to have a go at it yourself.
Taking proper advice from a professional is very important. If your painting is valuable then you will be advised to have it professionally treated. But if the painting doesn't warrant the cost then then you may consider using a conservation liquid, which is a mild solvent. It is designed to clean and remove varnish from your painting, and is applied using a cotton wool ball to smear the mix over the painting. Some people may be confident enough to remove the painting from the frame first, and this is advisable if you don't wish to risk any discoloration to its surround. Always apply a test patch first to test the reaction of the solvent to the canvas.
An application of 3 coats of natural picture Damar varnish should be applied after the old varnish has been stripped. Each coat of varnish should be allowed to dry for at least four days, with the last coat being applied after an interval of six months.
The cleaning and restoration of old paintings is not a job to be taken lightly. Always do a test area first with your chosen method of cleaning and only proceed with the job if you have the utmost confidence. Always remember to get a professional opinion as to the worth of your painting, and also be aware of the fact that by cleaning any valuable painting, nice though it may look in the long run, will probably become considerably devalued by the restoration.