Take a photograph of the painting so that you have a record of what it looks like should you make any mistakes.
Lay the canvas face up.
Dab the neutralizer on a piece of cotton and wipe the affected area very gently. The neutralizer will remove dirt that has accumulated on the surface.
Apply the emulsion onto a cotton swab and gently dab the coffee stain. Check the swab to make sure you are not removing paint before dabbing again.
Varnish the area you have cleaned with a soft brush.
Compare the cleaned canvas to your photograph and store the photograph for future reference should you stain it again.