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How Caulking and Sealing Protect Your Paint Between Repaints

By pasadena house painting journal

Paint gets the credit for protecting a house, but caulk does a large part of the work. Every place where two materials meet, such as a window frame against siding, trim against stucco, or a board end against a corner post, is a gap that water can enter. Caulk fills those gaps with a flexible seal so that rain runs off instead of soaking in. When caulk fails, water gets behind the paint at exactly the places where it can do the most damage. Keeping joints sealed between repaints is one of the most effective maintenance tasks a homeowner can take on, and one of the easiest to do well.

Why caulk fails in Pasadena's climate

Caulk ages the same way paint does, and for similar reasons. Ultraviolet light breaks down many sealants, making them hard and brittle. Heat speeds that process. Materials on either side of a joint also expand and contract with temperature, and in a climate with hot days and cool nights, joints open and close constantly. A caulk that has lost its flexibility cannot keep up with that movement. It cracks along the middle, pulls away from one side, or shrinks into a thin line that no longer fills the gap.

Sealing gaps is one of the cheapest maintenance jobs described on pasadena house painting journal, and one of the few a homeowner can check without tools.

Santa Ana winds add to the stress. Hot, dry wind pulls moisture out of wood trim quickly, causing it to shrink, and joints that were tight in spring can open noticeably after a wind event. The first rain after a long dry spell then finds every one of those openings.

Choosing caulk

Caulks are sold under many names, and the labels can be confusing. For painted exteriors, the main categories are these.

• Acrylic latex caulk. Inexpensive, easy to tool and clean up with water, and paintable. Basic versions have limited flexibility and can shrink and crack sooner than better products. They are fine for small, stable joints.
• Siliconized acrylic caulk. Acrylic latex with added silicone for better adhesion and flexibility, still paintable. A common choice for general exterior trim work.
• Elastomeric and high-performance acrylic or hybrid sealants. Formulated for more movement and longer life outdoors. Many are paintable and suit joints between dissimilar materials, such as wood trim against stucco.
• Polyurethane sealants. Tough and flexible, with good adhesion to masonry and concrete. Most can be painted once cured, but they are harder to work with and to clean up.
• Pure silicone. Very durable and flexible, but most types cannot be painted, and paint will bead off them. Avoid silicone on any joint you intend to paint, and be aware that silicone residue left behind after removal can prevent new caulk and paint from sticking.

Read the label for the joint width it is rated for, whether it is paintable, how long it needs before painting, and the minimum application temperature. A product rated for movement is worth the extra cost on joints between wood and stucco, which move differently.

How to spot joints that need attention

Check caulk during your spring and fall walk-arounds. Look closely, since failed caulk is easy to miss from a distance.

• Cracks running along the length of the bead.
• Separation from one side of the joint, often visible as a thin shadow line.
• Caulk that has shrunk into the gap so the joint looks concave or hollow.
• Hard, brittle caulk that crumbles when pressed with a fingernail.
• Paint peeling or discoloring along a joint, which often means water is getting in behind the caulk.
• Gaps where caulk is missing entirely, often at the tops of window and door heads.

Pay the most attention to the tops and sides of windows and doors, the joints where vertical trim meets horizontal surfaces, and anywhere wood trim meets stucco.

Removing old caulk

New caulk over old failed caulk rarely lasts. It bonds to the old bead, not to the materials on either side, and the old bead is already failing. Cut out the old caulk with a utility knife or a caulk removal tool, taking care not to gouge wood or stucco. Clean the joint of dust, loose paint and any residue. If the old caulk was silicone, clean the surfaces thoroughly, since silicone film prevents adhesion. On homes built before 1978, old caulk and the paint around it may contain lead, so avoid dry sanding and clean up carefully.

Deep joints may need a foam backer rod pressed in before caulking. Backer rod controls the depth of the bead and keeps caulk from sticking to the back of the joint, which lets it stretch properly between the two sides. A caulk bead bonded on three sides cannot flex and will tear.

Where caulking fits in the order of paint work

When you are repainting, caulking usually goes after cleaning, repairs and priming, and before the finish coats. Primer gives the caulk a clean, consistent surface to bond to, especially on bare wood. After caulking, the joint needs to cure for the time the manufacturer specifies before paint goes on. Painting too soon can cause the paint to crack as the caulk continues to cure and shrink.

For maintenance between repaints, the sequence is shorter: remove the failed bead, clean, prime any bare wood exposed, caulk, and then touch up the paint over the new joint once the caulk has cured. The painted caulk joint will blend better if the touch-up covers the full width of the trim rather than just the bead.

Application conditions

Caulk needs mild conditions to cure properly. Avoid application in direct afternoon sun on hot days, when the surface can be much hotter than the air and the caulk may skin over before it bonds. Do not caulk when rain is expected before the product has had time to set. Early morning in summer, or mild days in spring and fall, are best in Pasadena.

Sealing beyond caulk

Caulk is one piece of a broader sealing strategy. Nail holes should be set and filled before painting. Exposed end grain on trim, rafter tails and beam ends should be primed and painted carefully, since it absorbs water readily. Any penetration through the wall, such as a hose bib, electrical box, or pipe, should be sealed around its edges. Taken together, these small seals keep water out of the places paint alone cannot protect.

A few hours of caulking each year keeps paint lasting longer, protects wood from rot and stops the small leaks that turn into expensive repairs.