Matte and glossy passages in a paintings have always given artists problems. A few things you should know:
1. Adding linseed oil to the paint will increase the gloss.
2. Unevenly varying the ratio of linseed oil to paint will result in an uneven gloss.
3. Once the paint is dry it wont dissolve in Turpenoid.
The solution is to apply a glossy varnish coating to the entire painting after the paint is thoroughly dry in six months to a year. The gloss varnish should raise the gloss of the entire painting, and once its dry, you can apply a matte varnish to adjust the overall gloss.
Ross Merrill is chief of conservation at the National Gallery of Art in Washington, D.C.