Tree sap on house siding creates a sticky, unattractive mess. Homeowners may decide to simply paint over the area. Raw wood surfaces may ooze sap as well, making furniture crafters or house builders paint on top of it. Doing so will likely cover the mess but requires some preparation and thought about product choices.
Tannins
Tree sap creates a problem on painted surfaces because sap often contains tannins, acidic compounds also often present in bark, leaves, wood, roots and even fruits of various trees. Some species such as hemlock and the more common oak contain higher tannin levels. Dark pigmentation that bleeds through paint occurs from tannins, as well, often seen in cedar, redwoods and mahogany. Paint by itself will not block the acid in tannins, so the sap leaks through.
Cleaning
Clean sap off surfaces before painting to help reduce the chances of the tannins bleeding through and help create a smooth surface paint can adhere to. To remove sap, dip a rag in an oil-based cleaner, mineral spirits or a commercial bug and tar removal product. If the sap has hardened, leave the cleaner on the area until the sap softens, then wipe over the area until it is gone. Avoid using water to clean a wood surface since doing so can damage the wood.
Preparation
Effective preparation can help prevent further problems with sap. Make sure the section you cleaned the sap from is dry before proceeding. Brush off any debris from the area with a clean towel or brush. If the sap leaves an uneven area, use sandpaper to smooth it completely and then wipe off the dust. If the area appears oily, pour a small amount of turpentine on a rag and rub over the area.
Priming
To avoid the bleed-through of the tannins from the sap, prepare the surface with a primer before you paint. Primers containing alkyd/oil mixtures may seal the tannins in the wood. Common types of primers are labelled "universal primers" or "stain-blocking primers." A primer specifically designed for wood may be more effective but also cost more. If you are going to cover the wood with enamel paint, choose an alkyd/oil primer specifically formulated for an enamel coat for best results. When covering new wood, use wood sealer over knots in the pieces after applying the primer.