Depending on the type of paper (and more importantly the type of glue you use) the stain will probably attack the paper over a long period of time.
Isolating the stain is a good idea, and I can recommend the following approach.
- First give it a solvent-based coat of transparent varnish. This will protect the stain from the outside world and because of its similar composition will bond to it rather well. Let dry.
- Now give it a water-based coat of transparent varnish, this will enable the glue to bond. Let dry.
- Now use another coat of the transparent varnish and float the photo paper on it. Let dry
- Use one last coat of transparent varnish to encapsulate the photo.
A couple words of warning: test the water-based varnish to make sure that it doesn’t damage your photo and always ventilate!