Should I caulk the gap at the bottom of my stucco wall?

No. The metal strip at the base of a stucco wall is a weep screed, and the gap there is designed to let water that gets behind the stucco drain out. Sealing it traps moisture in the wall. It should stay open and clear of soil and mulch.

Can you paint over silicone caulk?

Not reliably. Paint beads up on pure silicone and peels off it, and silicone residue left on a surface can stop paint bonding nearby too. Where paint is going, the caulk should be a paintable acrylic, siliconized acrylic or elastomeric sealant. Old silicone in those joints is cut out and cleaned off first.

Is caulk applied before or after primer?

On bare wood, usually after. Caulk grips primed wood better than raw wood, which can pull moisture out of the sealant. So the usual order is wash and dry, repair, spot-prime bare areas, caulk, let it cure as the label says, then paint.

How wide a gap can caulk fill?

Caulk is made for joints, not holes. Most labels give a maximum joint width and depth, and deeper gaps need a foam backer rod pushed in first so the bead has the right shape and depth. Wider gaps or missing wood call for a repair, not a thicker bead.

Why does caulk crack after a year or two?

Usually because the wrong product was used for a joint that moves, the bead was too thin or too deep, or it was applied over dirt, old caulk or a damp surface. In inland heat, joints open and close every day, and only a sealant rated for that movement keeps up.

Caulking Before Painting: Where and What

Caulk belongs in joints that move a little and would otherwise let water in: trim against siding or stucco, the sides and head of window and door casings, butt joints and pipe penetrations. It does not belong in gaps built to let water out, such as stucco weep screeds, window weep holes and the bottom laps of siding. Paintable acrylic or elastomeric sealants go where paint goes; pure silicone does not.

A bead of caulk in the right joint keeps water out of the wall and makes the finished paint line look crisp. The same bead in the wrong place traps water inside the wall and causes the very rot and peeling it was meant to prevent. A good part of exterior preparation is knowing the difference.

What does caulk actually do?

It seals joints between two materials or two pieces that move slightly relative to each other, so that rain, irrigation and wind-driven moisture cannot get behind the paint. It also closes small gaps along trim so the paint line looks continuous. It is not a filler for holes or a repair for damaged wood.

Where caulk belongs

On the exterior, the joints worth sealing are the ones where water would otherwise run in:

the sides and top of window and door casings, where the trim meets stucco or siding;
vertical joints between trim boards and the wall, including corner boards;
butt joints where two lengths of trim meet end to end;
where wood fascia or trim meets stucco;
around penetrations: hose bibs, pipes, vents, light fixtures and electrical boxes;
small gaps along the edges of wood trim that have opened as the wood moved.

Inside, caulk is mostly about a clean line: the top of baseboards against the wall, crown molding, door and window casings, and the inside corners of painted trim. In kitchens and bathrooms, the joints where counters, tubs and showers meet the wall need a sealant suited to constant wet, which is a different product.

Where caulk does not belong

This is the part that gets homes into trouble. Some gaps on a house are there on purpose, to let water out. Sealing them traps it in.

The weep screed at the bottom of a stucco wall. This metal strip and its gap let water that gets behind the stucco drain away. It should stay open, and soil or mulch should be kept below it.
Weep holes in window frames. Many windows have small slots at the bottom of the frame that drain water collected in the track. They should never be painted or caulked shut.
The bottom edges of lap siding. The overlaps on horizontal siding let the boards shed water and breathe. Caulking the underside of each lap traps moisture behind them.
The drip edge under a window sill and the underside of trim that is designed to drip water clear of the wall.
Stucco control joints, the deliberate lines that let the wall move without cracking. They have their own sealing details and should not be smeared over with ordinary caulk.

A painter who caulks every visible gap, including these, leaves a house that looks tidy and fails from the inside.

Which kind of caulk goes where

Type	Where it suits	Paintable?
Acrylic latex	Interior trim, baseboards, casings	Yes
Siliconized acrylic	Interior and sheltered exterior trim joints	Yes
Elastomeric or hybrid sealant	Exterior joints that move, wood against stucco	Yes, per label
Textured stucco sealant	Fine cracks and joints in textured stucco	Yes
Pure silicone	Tubs, showers, glass, non-painted joints	No

The two rules that matter most: where paint is going, the sealant has to be paintable, and where the joint moves, the sealant has to be rated for that movement. Pure silicone fails the first rule, and plain acrylic fails the second on most exterior joints.

Why San Diego is hard on caulk

Inland heat. In El Cajon and the other valleys, a sunny wall can swing a long way in temperature between afternoon and the early hours. Wood trim and stucco expand and contract at different rates, and the joint between them opens and closes every day. A rigid caulk cracks along its length in a season or two.

Salt and damp near the coast. In Imperial Beach, a joint that has opened even slightly lets salty moisture in behind the trim, where it lingers. Caulk that has failed is often the starting point of the rot and peeling seen on coastal trim.

Older tract homes. On the post-war stucco homes of Chula Vista, old caulk is often dried out, shrunk away from the trim, or hard as plastic. Painting over it seals in the failure. It is cut out and replaced.

How caulking fits into the sequence

Wash and dry the surfaces. Caulk does not bond to dirt, chalk or salt, and it does not bond to a damp joint.
Cut out failed caulk and clean the joint. New caulk over old cracked caulk fails with it.
Make repairs to wood and stucco. Caulk is not a substitute for replacing a soft piece of trim.
Spot-prime bare wood, so the caulk grips primed wood rather than raw fibers.
Fill deep joints with backer rod first, so the bead has the right depth and shape.
Apply the bead and tool it so it is pressed into both sides of the joint.
Let it cure for the time its label gives before painting, within the temperature range on the label.
Paint, with the finish coats carried over the caulk.

Caulking in full afternoon sun, or late in the day near the coast when dew is about to settle, shortens the life of the bead. That is a scheduling question, not a product question.

What this means for your repaint

Caulking is detail work, and much of its value is invisible: water that never gets behind the trim, rot that never starts. Ebenezer looks at the joints during the repair check after the power wash, tells you which old caulk is failing, and leaves the joints that are meant to drain open. He uses Dunn-Edwards and Sherwin-Williams paints over a sealant chosen for the joint, not one tube for the whole house.

If the caulk around your windows has cracked or pulled away, or you are not sure what should and should not be sealed on your home, ask for a free estimate. Ebenezer will walk the house with you and point out each one. More about exterior painting and the full process.