The Ins and Outs of Caulking

Caulk is a pliable sealant. Caulk prevents air leaks that allow energy to escape and rain and snow to damage wood and insulation. Entering moisture can make a breeding ground for mold spores. Open spaces allow bugs and noise to enter. Find out more about caulking in our handy guide.

When you caulk before painting, your exterior painting project will last longer. Caulk eliminates the edges on gaps that allow water to seep under paint and start the peeling process. When you caulk before beginning your interior painting project your finished project will look neater and more defined.

With practice anyone can apply caulk in a neat ribbon or rope called a bead. If the first time you try your bead looks like sticky meandering goo with an exploded air bubble here and there, don’t give up. Learning to caulk is a skill necessary to on-going home repair.

Where to Caulk

A guiding principle is to caulk anywhere two different materials meet. For example caulk along house trim, where the window or doorframe meets the siding, where the siding meets the foundation, or where a pipe goes through a wall. Look for gaps around roof vents, skylights, and the chimney.

Do not caulk under the lap of lap siding. Interior moisture needs this space to vent. Without this space interior damage will occur and the interior moisture seeking an escape route will force off the paint on the siding.

Caulk bonds best when gaps are no wider than a quarter inch and when they are half as deep as wide. The caulk must adhere only to the joint walls and not to the bottom or rear surface. Use shims in wider spaces and a polyurethane foam backer rod in deeper joints. Push the flexible backer into the crack until it sits at least a quarter of an inch below the surface. Then fill with caulk. The backer rod will help support the caulk and prevent the caulk from sinking as it dries. The caulk does not adhere to the backer rod; it is adhering just to the joint walls. Backer rod is available in various sizes at hardware stores.

Because wood swells in our damp winter weather, exterior splits, cracks, and gaps are narrower in winter. When wood dries out in summer, gaps expand. A good time to caulk is in early fall and late spring when the space making up the gap is at its mid-point. The amount of expansion and contraction the caulk will go through will be reduced so it will last longer.

Some Common Caulks

Before WWII caulk was mostly just ground limestone mixed with soybean, tung, or linseed oil.
Today, caulks contain binders like silicone, latex, or polyurethane for flexibility and adhesion. Other ingredients include fillers to control consistency, additives like colorants and mildewcides, and water or solvents to hold the ingredients in suspension.

Latex caulk is the cheapest and easiest to use because it cleans up with soap and water. It is often used indoors between woodwork and wallboard and around window casings, baseboards, and doorjambs. But because latex caulk is water-based, it shrinks considerably as the water evaporates, making it not very flexible. Thus, caulk in a joint that moves or shakes will soon crack. Its service life is only five to 10 years.

Professionals prefer siliconized acrylic latex caulk for interior application because it has greater flexibility. In low stress joints it can last up to 20 years indoors or up to 10 years outdoors. It cleans up with soap and water. It is paintable in as little as one hour after application.

Avoid pure silicone caulk because while it is durable, it never dries completely and cannot be painted.

Professionals recommend polyurethane caulk for exterior application. It is the most expensive caulk but lasts the longest. It sticks to everything (including you), remains pliable, and can be painted. It is often advertised as life-long caulk with reports that it stays flexible for 50 years. If you don’t see polyurethane caulk displayed with all the other caulks, ask for it. It is usually hidden away in the contractor’s area of the store. You will need a solvent like mineral spirits to clean up when using it.

It is important to consider the life expectancy of the caulk, its shrinkage rate, whether it can be painted, and if it cleans up with soap and water or needs a solvent. Read the label before you buy any caulk.

Preparation

Apply caulk in the same weather conditions that you would apply paint. Apply the caulk when the surface and ambient temperatures will remain between 50 to 90F for the next 24 hours. Do not apply it in the rain or when the humidity is high.

Most sealants do not adhere to wet surfaces. They need a clean dry surface and affix better to primed surfaces than bare.

Always pry out old caulk before applying new caulk. A painter’s five-in-one tool or a stiff putty knife works great. For difficult-to-remove caulk use a caulk softening gel available at hardware stores.
Never try to pressure wash old caulk away. Water forced through gaps can cause serious damage.

Caulk Guns

Before the 1950s caulk was applied with a putty knife. Caulking guns became widely used with the appearance of disposable tubes containing caulk in the early 1960s.

Caulking guns use a plunger to push caulk out of the tube when the trigger is pulled. Most caulking guns are made of metal but fiberglass and nylon or ABS plastic guns are also available.

Look for a gun with a smooth plunger rod or a hex shaped plunger rod that moves effortlessly with light pressure on the trigger. Avoid caulking guns with a ratchet rod, which is difficult for a beginner to control.

To load the tube simply depress the plunger-release “tongue,” pull back on the plunger rod, and insert the tube nozzle-first into the gun. Pull the trigger until the plunger makes contact with the caulk.

Cut the tip of the tube off at a 45° angle as close to the tube’s tip as possible. Cut a small opening first. Use it for narrow gaps that need sealing. Enlarge the hole to fill larger gaps.

If the caulk comes in a cardboard tube, insert an awl or screw driver through the tip to puncture an inner seal to release the caulk. Plastic tubes of caulk usually do not have the inner seal.

Applying Caulk

Hold the gun at a slight angle similar to the angle of the cut tip. Insert the tip into the gap if you can; otherwise run the tip along the surface.

By pushing or pulling the gun slowly and evenly along the crack as the trigger is pulled, you will get a clean even strip of caulk. The cut tip of the caulking tube should force the caulk against the surface to be sealed.

When you come to the end of a seam, release the trigger and reverse the plunger rod slightly as you pull the gun away. Unless the plunger is reversed, the caulk will continue to seep out of the tube even without the trigger being pulled.

Smooth out the caulk application before it starts to dry. Latex or siliconized acrylic latex caulk can be smoothed with the back of a spoon dipped in slightly soapy water. Smooth polyurethane caulk with a spoon dipped in mineral spirits.

Less is better than more. If you find you did not add enough caulk, re-apply more before it dries. It is difficult to remove an excess of wet caulk. And it is impossible to even-out dry caulk. Dried caulk does not sand smooth. Instead it rips off like a stretched rubber band.

Storing Caulk

If you have caulk left in the tube after you have finished your project be sure to keep it fresh. Caps are sold to seal off the end of the caulking tube to prevent air from drying out the caulk. Wrapping the cut tip with masking tape or duct tape works well if the tube is also wrapped tightly in a plastic bag. Sealing the cut tip by sticking a big nail down the open tip is a bad idea because as you stick the nail in, the caulk oozes up around it and you have a nail that will be caulked in place and an unusable tube of sealant.

If the caulk should dry in the tip, carefully use a utility knife to split the tip. Remove the dried caulk then wrap the tip with duck tape to finish using the tube contents.

The plungers on most caulking guns do not extend the entire length of the tube so about three-fourths of an inch or more remains in the tube when it is thought to be empty. To get all the caulk out, remove the tube from the gun and insert a two to three inch length of dowel about the width of the tube into the caulk cartridge before inserting it back in the gun and bringing the plunger back into position.

Finally, all paintable caulk will benefit from being painted. Paint seals the edges of the caulk, adds UV protection to the caulk, and extends its durability.