Cyclopedia of Painting/Mildew
MILDEW.
Mildew is a serious trouble. This is a vegetable growth and always a sure indication of dampness. It is impossible to satisfactorily paint a surface on which mildew has formed unless the surface is first treated to destroy this growth.
Ochre primers and ochre colors are particularly liable to this really serious trouble, due to the fact that they are largely of bog origin and contain the seeds or spores as they are called from which the mildew mold develops. Such growths result not only in a most serious discoloration of the work which at times may be taken as fading or change of color, but also are very destructive to the paint itself, mildew not only developing at times at the expense of the vegetable oil itself, but what is even more serious, growing between the wood and the paint and thus forcing the paint off.
Vegetable oils like linseed oil are not destructive to this vegetable growth, but turpentine is, hence the first thing to do in aggravated cases is to wash well and freely with turpentine, removing any loose paint; this will very largely destroy such growths. In addition, an exceptionally large amount of turpentine should be used in the first coat applied over such a surface; the paint should be well flatted. An undercoating well flatted with turpentine applied over a mildewed surface which has been washed with turpentine offers the best possible protection against repetition of the trouble.