Every hardware store has an aisle that looks like a candy shelf for gaps. Tubes, tubs, cans, and cartridges, all promising to fill, seal, patch, or plug. Pick the wrong one and the result is anything from "looks a little off" to "water is now inside the wall." Pick the right one and nobody ever notices your work again, which, in this line of business, is the dream.
The trick is to stop thinking about the product and start thinking about the gap. Does it move? Does it get wet? Will you paint it? How big is it? Answer those and the aisle sorts itself out.
Caulk vs. sealant: the honest answer
In everyday speech, and on plenty of labels, "caulk" and "sealant" mean the same thing: a gooey material you gun into a joint that cures into a seal. If you ask for either at the store, you'll end up in the right aisle.
In the building trades, though, there's a real distinction:
- Caulk traditionally means the lower-movement, often paintable products, like acrylic latex and the old oil-based caulking compounds. They're great for filling cosmetic gaps that don't move much, especially indoors.
- Sealant usually means an elastomeric, higher-performance product designed to stretch and recover as a joint opens and closes: silicone, polyurethane, hybrid polymers, and friends. These are rated for how much joint movement they can handle.
The standards reflect that split. Latex sealants are covered by ASTM C834, while high-performance elastomeric joint sealants are specified under ASTM C920, which grades them by movement class. If a tube lists C920 and a movement class, it's a sealant in the professional sense, whatever the front of the label calls it. We decode all of that in the caulk aisle, decoded.
Bottom line: the word on the front matters less than the chemistry and specs on the back.
Grout
Grout is the cement-based (or sometimes epoxy) filler that goes between tiles. It locks tiles together, fills the spaces between them, and cures rigid. Sanded grout is for wider joints; unsanded is for narrow ones and delicate tile surfaces. Epoxy grout is tougher and more stain-resistant but trickier to work with.
What grout can't do is move. That's why the joints where tile meets a tub, a countertop, or another wall at an inside corner (anywhere two planes change direction) should get a flexible sealant instead of grout. Those are the joints that crack first when grout gets used. If you want the sealant to match the grout lines, use a color-matched sanded caulk like the one in our sanded caulk review.
Putty (two very different kinds)
Glazing putty
Traditional glazing putty is a dough of linseed oil and chalky whiting, used for centuries to seal glass into wooden window sashes. It firms up slowly, needs to be painted to last, and still has devoted fans among people who restore old windows. It isn't for modern vinyl or aluminum windows, and it isn't a general-purpose caulk.
Plumber's putty
Plumber's putty is a soft, non-hardening clay used to seal under sink drain flanges, strainer baskets, and some faucet bases, where it gets compressed by the fitting. It stays soft forever, which is the point. It's not for joints that are exposed, pressurized, or need to be glued, and some formulas can stain natural stone or aren't recommended for certain plastics, so use a stain-free putty or silicone where the fixture maker says to.
Spackle and joint compound
Spackle is a lightweight patching paste for nail holes, dents, and small cracks in drywall and plaster. Joint compound (mud) is its big sibling, used to tape and finish drywall seams. Both dry rigid and sand smooth, which makes them perfect for holes and useless for moving joints. Spackle the gap where the baseboard meets the wall and it'll crack along the seam within a season. That gap wants paintable acrylic caulk.
Wood filler and wood putty
Wood filler hardens and can be sanded, making it the choice for holes and gouges in bare wood that you'll paint (or carefully stain). Wood putty is usually oil-based, stays somewhat soft, and comes in colors meant to be pressed into nail holes after the wood is already finished. Neither belongs in the joint between two boards that expand and contract. That's caulk's job, or on exterior trim, a flexible elastomeric sealant.
Expanding foam
Spray polyurethane foam fills big voids fast: rough openings around windows and doors, pipe penetrations, rim joists, and gaps far too large for caulk. Use a minimal-expanding formula around window and door frames, because the regular stuff can push hard enough to bow a jamb. Foam isn't a finish. It needs to be trimmed after it cures and covered, since sunlight breaks it down. Some foams are sold specifically as fire-blocking for code-required penetrations; your local code decides where those are needed. For the gap that's too big for caulk but needs a finished bead, the answer is usually backer rod plus sealant. Our joint design guide shows how.
Side by side
| Material | What it's for | Once cured | Paintable? | Wet areas? | Don't use it for |
|---|---|---|---|---|---|
| Caulk (acrylic latex) | Gaps along trim, drywall, and interior joints | Firm, slightly flexible | Yes | Splashes only | Showers, big moving joints |
| Sealant (silicone, polyurethane, hybrid) | Joints that move or get wet; exteriors | Rubbery and elastic | Poly and hybrids yes; silicone no | Yes | Silicone on anything you'll paint |
| Grout | Between tiles | Rigid | Not usually | Yes, sealed | Corners, tub-to-tile, changes of plane |
| Glazing putty | Glass in wood window sashes | Slowly firms | Yes, and it must be | Only when painted | Vinyl or aluminum windows, general gaps |
| Plumber's putty | Under drain flanges and some faucet bases | Stays soft | No | Yes, under fittings | Exposed joints, some stone and plastics |
| Spackle / joint compound | Holes and cracks in drywall and plaster | Rigid, sandable | Yes | No | Trim gaps, corners that move |
| Wood filler | Holes and gouges in wood | Hard, sandable | Yes | Exterior formulas exist | Joints between boards |
| Expanding foam | Large voids and rough openings | Rigid or semi-rigid foam | Once trimmed, but cover it | No | Visible finished joints, sun-exposed spots |
So which one does your gap want?
- A hole in a wall? Spackle.
- A hole in wood? Wood filler if you'll paint, wood putty if it's already finished.
- The space between tiles? Grout.
- Where tile meets a tub, counter, or another wall? Flexible sealant, usually 100% silicone or color-matched sanded caulk.
- A dry, painted interior joint? Acrylic latex caulk.
- An exterior joint that moves? An elastomeric, polyurethane, or hybrid sealant.
- A gap bigger than your pinky? Backer rod plus sealant, or foam if it'll be hidden.
- Under a sink drain? Plumber's putty or silicone, per the drain maker.
Still torn? Tell the Caulk Finder where the gap is and it'll point you to a type and a tube.