Filling cracks and holes with caulk in your home's trim and siding before painting not only makes the paint job look better, but also makes the paint last longer. Cracks and holes in any surface collect water, which causes paint to peel.
Caulk is a substance designed to seal a joint between two surfaces and to fill small holes. Use top-quality caulk outdoors, where it needs to withstand extreme temperature changes and remain flexible for 30 to 50 years. Before caulking joints between dissimilar materials, make sure that the manufacturer recommends the caulk for both materials. If you're going to paint the area, the caulk must be paintable. If you're unsure whether a caulk is suitable for a particular application, contact the manufacturer's customer service department.
The best time to caulk is after you scrape, sand, and prime. Caulk adheres better to primed surfaces, and the gaps, cracks, and holes are more evident.
Scrape away any peeling paint adjacent to the caulked areas. If doing so exposes bare wood, recaulk all cracks between any two nonmoving materials. Let the caulk cure for a few days before power-washing the exterior.
Don't caulk the horizontal joints on siding where the siding courses overlap. The cracks between two courses of siding provide ventilation points to let moisture escape from the siding and from inside the wall. In fact, one of the often-recommended cures for a moisture problem involves inserting numerous wedges between siding courses to create a larger gap through which moisture can escape. For the same reason, don't try to fill the joints between courses with paint.
Fix small holes in siding with an exterior patching compound, available in a premixed form (much like interior spackling compound) and in a dry powder form that you mix with water. Just make sure the package states that the patching compound is for outdoor use.
To patch holes and depressions in siding of any kind, follow these steps:
Clean the hole or depression you plan to fill.
Roughen the area you want to patch with 80-grit sandpaper for a better bond.
Fill the area with patching compound.
Apply the compound to the hole or depression in one direction and then smooth it in a perpendicular direction so that it's level with the original surface.
Allow the compound to harden and then sand it smooth.
Compound shrinks, so a second coat may be necessary. Be sure to remove the dust from sanding the compound before you apply a second coat.
To repair large cracks and damaged trim, remove any rotten wood with a chisel. Use a two-part polyester-based compound, similar to auto-body filler, to make the repair. Two-part fillers come with a thick paste base and a small tube of hardener. Mix the hardener with the paste according to the manufacturer's directions. The filler sets up within three to five minutes, so mix only as much as you can use right away, and clean your tools immediately after use.
Use a putty knife to apply the compound to the damaged area and level it with the surface. This filler doesn't shrink as much as premixed exterior fillers do, but you still may need to apply several coats to fill a large hole. When the filler hardens, it's suitable for rasping, sanding, or drilling.
Use auto-body fillers or two-part polyester-resin fillers for aluminum siding in a similar way. But before you apply the filler, drill numerous 1/8-inch-diameter holes in the patch area. When you apply the compound, it locks into the holes for a better bond. You can also cover damage with a new piece of siding. Just cut the top flange off so that the patch fits under the course above and embed it in adhesive caulk along the top and sides.
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