First make sure your painting is completely dry. Allow several months for an oil painting to dry properly. Depending on the thickness of the paint, this could be up to nine months.
Clean the painting so it's free from dust, dirt, and grease. Lay the painting flat, then dampen a sponge and gently clean it.
Let this dry.
Use a Soft flat bristle or foam brush to apply the varnish. Use a matt varnish if you don’t want your painting to be glossy.
With the painting flat, work from the top to the bottom, applying the varnish in parallel strokes from one edge of the painting to the other. Always work in the same direction. Hold the painting up so you can see over the whole surface to make sure you didn’t miss a spot.
When the first coat of varnish is dry, apply a second coat at right angles to the first. This will give you a good, even finish.
Leave the painting flat for at least 10 minutes after you've finished varnishing to prevent the varnish running.
Then prop it up against a wall to dry, face inwards.
It should dry within a day or two, depending on the weather if it is humid it will take longer. Test the varnish to see if it is dry by touching the edge of the painting. If it feels tacky it is still wet.
Always varnish the whole painting at once. You don’t want to do part of it one day and the other the next day you may see a line where you stopped and started.
If you try to apply the second coat before the first is dry it may “pull” the first coat off.
Make sure you have the same amount of varnish on the brush for each stroke so you put equal amounts of varnish on all parts of the painting.
Work in a dust-free environment, or else dust particles will get stuck in the wet varnish. Keep cats and dogs out too, animal hair seems to gravitate to a newly varnished paintings!