When repainting over multiple coats of paint, it is realistic to expect something less than the results achievable when painting a new surface. This is especially true when there are several coats of paint of unknown compositions or qualities. Surface preparation at the time of the original paint job or during subsequent paint jobs may also cause concern.
Some of the problems which manifest themselves following a repaint can be a direct result of the original preparation or quality of paint used. Peeling and mildew are two of the most common disappointments consumers encounter. The source of these problem can be hidden between coats of old paints, or even exist at the original surface.
Mildew is one of the more pervasive problems. There is a misconception in construction that newly applied wood siding or trim is free of mildew and, therefore, doesn't need preparation before priming and painting. Coating over mildew will not contain or kill it. It will continue to manifest itself beneath paint until it finds an outlet through migration or until it grows to the extent that it destroy the adhesion of the original coat of paint. Peeling and flaking may ultimately occur.
Old solvent-based paints applied to wood will also eventually fail as the paints age. Solvent-based paints lack the breathability and flexibility required for long-term adhesion to wood. As they age, they embrittle. As the brittle paints crack along the grain of the wood, moisture may enter and mildew achieves a foothold. Escaping moisture leads to flaking, which exposes more raw wood to mildew and moisture, and the problem is compounded.
Topcoating solvent-based paints with flexible, breathable, mildew resistant acrylic latex paint sometimes actually aggravates a flaking problem. As the acrylic latex contracts upon drying, it often loosens the already tenuous adhesion of the solvent-based coating to the surface. Peeling may occur which exposes the bare surface.
When dealing with older buildings, the purist approach is to recommend complete removal of all previous coatings down to the original substrate. This is rarely practical or affordable, so compromises are made. As a rule of thumb, the more old paint that can be removed to the bare surface, the better the odds for satisfactory performance when repainting.