Pores, Dimples and a Ghost Seam: Why Nitro Finishes Sink Into the Grain

You tilt a Les Paul toward the window and the back isn't glassy anymore. There's a fine field of tiny dimples following the mahogany grain, and maybe a faint line running down the center seam that wasn't there when the guitar was new. Players on the forums ask about this all the time, usually right after they notice it: is my finish failing? Was it a bad factory job? Is it a repair?

Short answer: on a nitrocellulose finish over an open-grained wood like mahogany, some grain sink over time is normal. It's cosmetic. It doesn't touch tone or playability, and on a used guitar a few years old, mild sink shouldn't cost it much value. It's also worth telling apart from the things that look similar and do matter. Here's what's going on and how I'd judge it on a used guitar.

What "finish sink" actually is

Mahogany, ash and rosewood are open-pored woods. Look closely at raw mahogany and you'll see tiny channels running along the grain. Maple, alder and basswood are closed-pore and much smoother. To get a glassy finish over mahogany, the builder first packs those pores with grain filler, then sprays sealer, color and clear coats, then sands and buffs the surface flat.

Nitro doesn't cure the way modern poly finishes do. It dries as its solvents evaporate, and that goes on long after the guitar leaves the factory. As the lacquer loses solvent it gets thinner, and it follows whatever is under it. Wherever the filler didn't fully level the pores, or settled a little itself, the surface slowly settles into each pore. The finish was flat on the day it was buffed. A year or three later, you can see the grain as texture, not just color.

That's why finishers commonly advise letting nitro cure for weeks, often a month or more, before the final sanding and buffing. Buff it too soon and you level a surface that's still moving. Nobody outside the factory knows exactly how long a given production guitar cured, so I won't guess. But noticeable sink in the first year or two fits either a fast schedule or a deliberately thin finish.

The "ghost" center seam

The second version of this is a faint line or shallow trough right over a glue joint: the center seam of a two-piece top or back, or the joint where a wing was added. Woodworkers see the same thing on glued-up furniture panels. The moisture in the glue swells the wood right at the joint. If the panel is surfaced and sanded before that moisture leaves, the joint is flat when it's finished. Then the wood dries back and pulls in along the seam. Add a nitro finish that is shrinking too, and a seam that was invisible for years can start to show in raking light.

A ghost seam isn't a crack. You might feel it with a fingernail as a shallow valley, but the finish is continuous. A crack has a sharp edge, often with finish lifting along it, and it usually wanders instead of following a straight glue line. If you can't tell, shine a flashlight across it from a low angle, or ask a tech.

Seeing grain vs. feeling grain

Buyers often lump two things together. Seeing mahogany grain through a translucent finish is by design. Seeing the surface follow the grain is sink.

When I inspected that Les Paul Custom, I noted that the Wine Red was "deep and rich without hiding the mahogany grain underneath." That's the look you're paying for with a translucent finish. To check for sink, stop looking straight at the wood. Turn the guitar so a light source reflects off the back. On a level finish the reflection looks like a clean, unbroken window. With sink, the reflected edges go wavy and you can see the pore lines as a pattern in the gloss itself.

Thin, lightly tinted finishes show grain the most. Examples are the VOS-style Custom Shop finishes, TV whites and yellows, and satin guitars. That's usually a big part of their appeal.

The Brian Ray SG Junior is a good example. The White Fox finish is pale and pearlescent with the grain visible underneath, and I noted in the listing that it keeps the guitar "from feeling overly polished or sterile." Some players see texture and think it's a flaw. Others buy a guitar because of it.

Is it a defect? My take

This is where the online arguments start. One side says a $3,000+ guitar shouldn't look like orange peel after two years. The other says that's what real nitro does, so buy poly if you want glass forever. Both have a point. Mild, even sink over the pores of an older nitro guitar is normal aging. I don't treat it as a flaw in a condition grade any more than light finish checking. Heavy sink on a nearly new guitar, where the pores look barely filled, is a legitimate cosmetic complaint about how that guitar was finished. It still doesn't affect how the guitar plays or sounds, so how much it bothers you is up to you. What I wouldn't do is pay mint money for a guitar with obvious sink and then act surprised later.

Thick modern poly finishes cure chemically and don't keep shrinking the way nitro does, so they rarely show this. (The full debate is in Nitro vs. Poly: Does It Change Your Tone?)

What should worry you instead

On a used guitar, sink matters mostly because of what it tells you about the finish's history. Here's how I read it:

• Even sink across the whole back and sides is normal aging of the original finish, and it's actually a mild sign the finish is original.
• A patch that's glassy flat while the rest has sunk suggests someone worked that area. It could be a touch-up, a drop-fill, or a respray over a repair. Look for a halo, a sheen change, or a color shift at its edges.
• Zero texture and an unusually thick, glassy build on a guitar that should be thin nitro can point to an overspray or refinish. On a refinished 2007 Les Paul Studio Premium Plus we had, I noted that the serial number was "considerably harder to see beneath the finish than it would be from the factory." Extra finish buries details. See How to Spot a Refinished Guitar.
• A line that isn't on a glue joint, or that has a sharp edge or lifting finish, isn't sink. Treat it as a possible crack until you know otherwise.

Can you fix it, and should you?

Nitro is easy to repair because each new coat partially melts into the one beneath it. A finisher can level-sand and buff a sunk area, or add clear coats and then level. Done properly, it looks factory-flat again.

On a collectible or nearly new Gibson, I'd usually leave it alone. Factory nitro is often thin, and leveling means cutting down to the bottom of every dimple, which is how people burn through to color on edges and carves. Any work also changes the answer to "is the finish all original?", which usually matters more on the used market than a little texture.

If you're thinking about value, the kind of sink described above isn't something I'd take much off for when making an offer. Overspray and refinishing are what hurt value. If you're selling or trading, our Cash Offer page is the quickest way to get a real number. And if you want a nitro guitar whose finish has already done most of its settling, have a look at what's in stock right now.

FAQ

Will the sinking keep getting worse?

Usually it's most noticeable in the first year or two while the lacquer is still losing solvent, then it slows way down. Nitro keeps changing slowly for its whole life, so an older guitar can keep developing texture, but it's gradual. Nobody can give you an exact timeline for a particular guitar.

Does finish sink affect tone?

No. We're talking about finish depth differences you can barely feel with a fingernail. Anyone telling you a sunk finish means the guitar is "opening up" or "breathing" is making a leap I can't back up.

Can I polish it out myself?

Polish takes out haze and light scratches but won't level dimples. Leveling means sanding, and on a thin factory finish that's a job for someone who has done it before. Cutting through to color on a Les Paul's carved top is a much worse problem than a textured back.