Q: My daughter and I bought an 1880ish Victorian and had the original hardwood floors in the kitchen refinished. We love the character of the dark marks the nails have created over time, as the floor is directly over a dirt crawl space. However, because this is a kitchen floor, lots of things are dropped and end up in the gaps, and the floor is then difficult to clean. Is there a way to seal or otherwise close the gaps?

A: Installers of new wood floors spread a puttylike goop across the wood, scraping it into any cracks with a stiff scraper. They then sand and finish the floor. But filling gaps between boards in a floor like yours is more complicated.

With new wood flooring, installers can cover the floor first with a moisture barrier to keep water vapor from moving up from the soil and into the wood. That’s important, because wood, no matter how carefully it is dried or installed, will always continue to expand and contract as relative humidity rises and falls with changes in the weather. That repeated expansion and contraction can push out whatever filler you add.

Tom Salisbury, owner of Salisbury Woodworking, a wood flooring installer in Poulsbo, Washington, with decades of experience working on vintage floors and installing reclaimed wood to create looks similar to what you have in your kitchen, suggested that you should address your dirt crawl space before you work on your kitchen floor. “When wood flooring is directly over a crawl space, make sure that all exposed dirt in the crawl space is covered with 6 mil poly sheeting to help keep excessive moisture out of the underside of the flooring,” he said in an email. “This will help stabilize the wood and the spaces between the boards.”

Along with spreading the plastic, also consider sealing air vents into the crawl space and insulating the walls, a process known as “crawl space encapsulation.” It would effectively make the crawl space part of the heated and air-conditioned part of your house and would make your house more energy-efficient and less drafty, in addition to helping to stabilize the floorboards. The federal Energy Star program offers advice, which you can find by typing “Basement & Crawlspace Air Sealing” into a Web search tool, such as Google. Basement and crawl space encapsulation companies also are set up for doing it, though it’s not cheap. NV Waterproofing & Foundation Repair in Manassas, Virginia, said the price typically ranges from $5,000 to $15,000, depending on the space.

When installers put in a new wood floor, they have the option of adding a moisture barrier under the flooring itself. But on an existing floor, that’s obviously not an option. And when the installers spread a filler to plug gaps between boards, they have a few advantages you don’t have. The wood is clean, so the filler sticks better than it might on a floor where the gaps are full of gunk. Also, they can spread the filler before the final sanding and application of finish.

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In your case, you might want to avoid doing anything that requires resanding and refinishing. The nail stains you see could be a sign that the floor is already sanded down about as far as it can go before the nails themselves begin to show. And there’s the complication about what color to choose for the filler. On a new floor, installers just need to match it to one of the colors in the wood. You’re dealing with both the wood color and the dark stains. If you match the filler to the wood, you could wind up with light-colored plugs at each nail hole, surrounded by a halo of dark wood.

Salisbury said that if you do want to proceed, he suggests scraping out the spaces between boards as best you can. A flexible putty knife, a stiff brush and a vacuum might be helpful. Then he suggests that you mask off the edges of the boards but leave the gaps exposed. Painter’s masking tape, often blue, would work for this. With a plastic putty knife, you can then spread gap filler in the openings and smooth it level. Remove the tape and you’re done, with no need for refinishing and no smearing of caulk over the finished floor.

Choose filler that’s color-matched to the finished wood or consider using a black filler, which would give you a look similar to what you have now and avoid the awkward plugged look where the nail stains are.

Salisbury recommends Color Rite acrylic caulking. It’s durable yet remains flexible, he said, and it’s suitable for applying to a floor that already has finish on it. “Most wood flooring fillers are hard and not flexible,” he said, “so when the flooring shrinks during the heating season, the filler pops out.”

Salisbury also guesses that you don’t actually have a hardwood floor. After seeing the pictures you sent, he said he’s pretty sure the wood is Douglas fir, a wood he knows well. In the early 1880s, the Port Blakely mill on Bainbridge Island, Washington, where he lives, was shipping out 200,000 board feet of it a day, making it the biggest sawmill on the Pacific Coast. Straight-grained, stable and harder than some so-called hardwoods — even though it is technically a softwood — old-growth Douglas fir is found in the floor and trim of many Victorian houses and those built in later decades.