A bead of caulk properly applied at a window frame joint takes three minutes. Left undone, that same joint admits water during every driving rain event, lets it channel behind the siding, and creates the wet substrate conditions that produce peeling paint, wood rot, and eventually framing damage. Exterior caulking is where paint jobs either hold through hard weather or begin failing the first season.

The caulk applied before an exterior paint job is not a cosmetic step. It is the moisture management step. Paint protects surfaces from UV radiation and abrasion. Caulk controls where water can and cannot enter the building envelope. Both functions are necessary. Neither substitutes for the other.

Which Joints, Gaps, and Cracks Need Caulk Before Painting

Not every gap in an exterior wall needs caulk, and applying caulk in the wrong locations causes problems. The selection of which joints to fill is based on whether the joint is a water entry point or a designed drainage path.

Caulk these locations:

Window and door frames where they meet the siding are the primary caulking locations on most homes. Water that contacts the siding above a window runs down the face and finds the joint between the frame and the siding. A gap of even 1/16 inch at this joint allows significant water intrusion during a sustained rain event.

Trim boards where they butt against siding have end grain exposed at the joint. End grain absorbs moisture at two to three times the rate of face grain, and any gap at these joints accelerates this absorption. Caulk these joints thoroughly before priming.

Electrical conduit, light fixtures, hose bibs, and any other penetrations through the exterior wall need a full bead of caulk at the point where the fixture or conduit meets the siding or trim. These circular penetrations are difficult to seal, and a gap around a hose bib sends water directly behind the siding.

Corner boards at inside and outside corners of the house collect water running down the face of the siding. Any separation between the corner board and adjacent siding creates a gap at a high-water-traffic location. Fill these.

Horizontal trim boards and belt courses have a top edge that collects water on its horizontal face. Caulk the joint between the top face of the trim and the siding above it to prevent water from running into the joint by gravity.

Do not caulk these locations:

The bottom edge of lap siding boards should never be caulked. This gap is the drainage path for water that gets behind the siding. Sealing the bottom of lap joints traps water that finds its way behind the siding and creates the wet conditions that cause rot and peeling.

The bottom edge of trim boards should also be left unobstructed to allow water to drip free rather than being held against the siding.

Masonry weep holes in brick veneer and CMU walls are drainage openings by design. Filling them with caulk causes catastrophic moisture accumulation inside the wall.

Choosing Paintable Exterior Caulk That Lasts

The single most costly caulking mistake is using interior-grade or silicone caulk on exterior applications. Silicone caulk does not accept paint. A bead of silicone caulk applied at a window frame before painting results in a paint line that permanently shows the caulk joint as an uneven, non-adhering edge. Removing dried silicone from exterior surfaces is extremely difficult and often requires mechanical removal that can damage the surrounding siding.

For exterior applications, the professional standard is a high-elongation 100 percent acrylic caulk. Sashco Big Stretch is the top professional choice for exterior applications requiring flexibility. It stretches 500 percent or more without cracking, has a service temperature range of negative 30 degrees Fahrenheit to 250 degrees Fahrenheit, and is paintable with latex paint after four hours. It is paintable with oil-based coatings after one week. Its VOC content is 59.8 grams per liter and it adheres to vinyl windows and siding without primer.

The critical test of a caulk’s exterior suitability is its elongation rating. Exterior joints move. Wood siding expands and contracts with moisture and temperature changes. Window frames move differently than adjacent siding. A caulk with low elongation, in the 25 to 50 percent range, cracks as the joint moves and begins admitting water within one to two seasons. Sashco Big Stretch’s 500-plus percent elongation means it accommodates substantial joint movement repeatedly without failing. This is why professional painters consider it significantly superior to budget exterior caulks and interior-grade siliconized acrylics like DAP Dynaflex 230 for any application that experiences weather exposure.

For joints deeper than one half inch, insert a backer rod before applying caulk. Backer rod is a foam cylinder pressed into the joint to provide a backing surface at the correct depth. Caulk applied without backer rod in deep joints makes three-point contact: it adheres to both sides of the joint and to the back of the joint. Three-point adhesion prevents the caulk from stretching when the joint moves and causes premature failure. Caulk over backer rod adheres only to the two sides of the joint, which is the correct configuration for flexible performance.

How to Apply Caulk for a Clean Professional Line

Caulk application technique determines whether the joint looks clean and professional or lumpy and inconsistent. The difference comes down to bead size, tool angle, and tooling method.

Cut the caulk tube tip at a 45-degree angle. The cut diameter determines the bead width. For most trim and window joints, a bead diameter of 3/16 to 1/4 inch is appropriate. For wider joints or deep cracks, increase the tip opening incrementally. Cutting the tip too wide results in excess material that is difficult to tool cleanly.

Use a smooth-rod caulk gun rather than a ratchet gun for better flow control. Ratchet guns dispense in incremental clicks that make it difficult to maintain a steady bead. Smooth-rod guns dispense continuously and release pressure when the trigger is released, stopping the bead immediately. Dripless guns with a pressure-release button are the preferred type for overhead and vertical applications.

Hold the gun at a consistent 45-degree angle to the joint with the tip facing the direction of travel. Push the gun along the joint rather than pulling it. Pushing presses the caulk into the joint ahead of the tip. Pulling drags the tip across the surface of the caulk and can lift it out of the joint rather than pressing it in.

After the bead is laid, tool it within one to two minutes before a skin forms. Tooling smooths the bead surface, presses the caulk into the joint for better adhesion, and creates the concave surface profile that sheds water. The simplest tooling method is a moistened finger or a caulk tooling spoon. Draw the tool along the bead with consistent pressure and a single continuous pass. Stopping and starting in the middle of a long bead creates tool marks.

Wipe excess caulk from the surrounding surface immediately with a damp cloth before it skins over. Caulk dried on siding or trim requires scraping to remove, which risks scratching the surface.

Caulking Around Windows, Doors, and Trim Joints

Windows and doors require more careful caulking attention than other exterior joints because of the volume of water they direct toward the building envelope and the number of distinct joint types involved.

The perimeter joint between the window frame and the surrounding siding or trim is the highest-priority caulk location. This joint sees water running down the face of the house after every rain. Apply a continuous bead at the interface between the frame and the siding or trim, pressing the caulk into the joint completely. Ensure the bead bridges the full gap and adheres to both surfaces.

The joint between the window trim and the siding below the window sill needs particular attention. Water runs down the face of the window and collects at the sill. If the sill’s perimeter is not fully caulked, this collected water enters the wall assembly at the base of the window, which is exactly the location where wall rot most commonly begins.

For door frames, the joint at the threshold and between the door frame and the siding above the door are critical. The joint above the door frame is often covered by a drip cap or flashing, but any gap between the flashing and the siding behind it should be caulked from the face.

Trim board joints where pieces of trim butt together, such as at the corners of casings or at splices in long horizontal trim runs, need to be caulked and, on wood trim, primed before the topcoat. End grain exposed at trim joints absorbs moisture aggressively. Fill these joints, allow the caulk to cure, prime the area, and then topcoat. This sequence seals the joint completely and prevents the gradual moisture infiltration that causes trim paint to fail at joints before it fails anywhere else.

Allow all caulk to cure to the manufacturer’s specified minimum before priming. Sashco Big Stretch accepts latex primer after four hours in normal conditions. Apply primer only over fully cured caulk to avoid wrinkling or adhesion problems at the joint.