Sanding: When is enough?

[TwinDad]

I sanded the neck of my kit guitar this evening. I started with 180, then did 220, 320, and 400 dry. It went surprisingly quick... less than 45 minutes all told, hand sanding.

I realized something... I don't really know when I've reached "enough" with each layer of sanding. I guessed by feeling the neck with my hand, working until I felt uniform smoothness with no discernible (relative) rough spots. Still it all seemed to go pretty quick. It started off pretty smooth for unfinished, but still...

Is there a better method, rule, gauge, etc. for determining when you're ready to move to the next grit?

 

[Abominable_Bass]

Do you plan on finishing/seal the neck? Or do you want natural wood smooth?
Generally you move to the the next grit once all sanding marks from the previous grit are removed.
Imagine lines in the sand and slowly filing those lines until smooth. If you don't fill all lines evenly the sand will never be smooth but will be bumpy.
Even your fingers will fool you into thinking something is level when you have low and high areas. A ruler or straight edge used to make sure you are not sanding to deep in areas is a great idea, also a depth gauge is good for making sure you don't have humps.

 

[TwinDad]

I do plan on clear coating it. Haven't decided exactly with what yet... reading up on different options.

 

[CryingBass]

Yeh, it's like Prunes: Is six enough? Is twelve too many?

 

[Rôckhewer]

There isn't a necessarily a "standard" answer to your question.
Variables such as,
the species of wood in question,
The surface condition prior to sanding ,
The type, or quality of sandpaper used,
The amount of pressure, and if you are using a backer ....
(When sanding a neck that is flat...if you want it to stay flat, or if you need to flatten high spots or sand down to even out a low spot ... you should be using an apropriate backer, or block)....
Your hand alone is soft, and as a backer...wil conform to irregularities. Magnifying, rather than removing them.

If the neck is flat (straight, consistent)....and is maple , which I would imagine would be typical for a kit..
You will probably be ok by hand starting a 180g as long as you sand lightly, using long strokes with the grain.
Depending on the surface condition prior, ... I am assuming there was deeper scratches or milling/ file marks you are trying to remove.
You need to look closely at what you are accomplishing as you sand.
There is no requisite time or number of strokes.
Stop and blow off the dust, frequently.
Make sure you can tell the difference between scratches, and the grain itself.
When you have removed the larger previous scratches...stop, move to the next higher grit.
If you can't see well , make sure you can.
Use good lighting. Don't trust feel alone.
The surface may feel smooth, but you can easily leave behind prior grit scratch.
A flashlight held at a low angle helps.
Wetting with a non grain raising liquid such as naptha or mineral spirits can show you scratches...but I personally don't like doing that in between grits... too much time wasted waiting for it to dry.
Just look closely... or use a magnifying glass.
If in doubt, take it outside in a bright area, but not necessarily direct sun. Natural light will expose a lot.

Sorry for the long post.
When I was a woodshop supervisor, guys who regularly over sanded, drove me nuts. It is very easy to distort an erstwhile flat surface by oversanding.

 

[Gilmourisgod]

+1 on the low angle raking light, keep turning the piece and look from multiple angles, sometimes things you missed will pop out. I always seem to miss something someplace, but keep at it.

 

[Englishman]

+1 on the taking it outside. I always think I'm done until I do that and all of a sudden it's scratch city.

 

[Jisch]

All good advice above, I don't think I've ever put the first coat of finish on and NOT found a flaw. I would also say don't go with a grit finer than 320 or so otherwise you're burnishing, not sanding. It's not such a problem on the neck, but if you're doing a natural finish on a body you don't want to start on burnished wood.

 

[Beej]

Good stuff here, I'll definitely second using a light at an oblique angle to make those tiny scratches cast shadows and expose their own nefarious intentions. Only other thing I'd add is that you can also use a tack cloth to wipe back the surface as you go - that will also expose scratches as it removes the fine dust that could be laying in the scratch grooves.

Also, if you have bad eyes and/or are old like many of us, grab a set of reading glasses to help you see those tiny details...

 

[TwinDad]

Thanks for the advice so far, folks. I took the neck outside this morning and examined it closely, including shining a flashlight on it at low angles... I did find one rough spot on the top edge of the headstock that needs some work. The rest looks pretty good, though I'll have another look or two before declaring it done. I do need to go over it with the tack cloth, and the reading glasses are a given.

 

[GMC]

Also, clogged high grit sand paper and easily put some deep scratches back into the surface that look like missed grits.

 

[roller]

I do plan on clear coating it. Haven't decided exactly with what yet... reading up on different options.

Tung oil makes for a great finish on necks and bodies. A little provides a nice, raw feel... and the more you apply, the shinier, slicker it gets.

Wait 24 hours between coats. Some folks sand with a light (800-1000) wet sandpaper in between the coats.

10 coats will give a maple neck the feel and look of a glossy finish... but way less tacky.

 

[T_Bone_TL]

Despite saying there isn't standard answer, @Rôckhewer slips what I consider to be the standard answer in there.

Enough to remove all scratches of the previous grit. No less, no more. Or at least not much more.

You also need to clean up between grits to prevent any loose particles of a previous larger grit from making new scratches when working with a finer grit.

Use a magnifying glass if need be as well as the strong side light. Or reading glasses you don't need yet (or stronger ones than you do need yet.)

I actually understand the process a lot better as a side effect of learning to polish fiber optics, where the grits are finer, the magnifying glass is a microscope, but the idea is pretty much the same, really - you progress by removing the prior scratches. Overdoing it is bad for different reasons, but still bad.

 

[I-Am-The-Slime]

don't go with a grit finer than 320 or so otherwise you're burnishing, not sanding. It's not such a problem on the neck, but if you're doing a natural finish on a body you don't want to start on burnished wood.

Interesting! Could you please elaborate? I'd like to understand what's going on there physically. I expect there may be differences between woods of different hardness or density...?

 

[Jisch]

From what I've read if you use finer sandpaper than 320 you are starting to burnish the wood, essentially closing up the grain. If you're applying an oil finish you want it to absorb into the wood. Burnishing prevents this. One of my first instruments I sanded to 800 grit before finishing, the tung oil definitely sat on top more than on subsequent instruments (after I read about this). I've never done an A/B comparison on test wood to prove what each would look like side by side.

 

[megafiddle]

Something that helps when moving to finer grits, is to use each finer grit at right angles to the previous grit.

This makes it easier to determine when the new sanding marks have completely replaced the old.

Although it's simpler to alternate directions on a flat surface, sanding crosswise on a neck can be done by looping long strips of paper over the neck and pulling the paper ends back and forth, working uniformily from one end of the neck to the other.

-

 

[Gilmourisgod]

I sanded mine up to 400 grit, and it generated wood dust, so I don't think that counts as burnishing, but that's on maple, I can see something softer like mahogany getting a little fuzzy. I like to dampen the grain a little one time after 220 grit, which raised the grain a little, not much point beyond that.

 

[Gopher]

My $0.02...

When I sand I have 1 of 3 possible goals.
1) Shape - this one is obvious
2) Smooth - this is obvious too for the most part but when I stop is dictated by the finish and the grain. Many good points have already been made about burnishing and oil finishes.
3) Clarifiy - this is a preference / nuance thing but IMO makes a big difference. As you sand higher and higher grits, the surface doesn't really get much smoother to the touch but if you watch the grain closely, you'll see it becoming gradually clearer like a picture coming into focus. I stop when I've achieved the clarity that I want which varies from species to species.

 

[JohnnyR]

One point not yet mentioned is the grit progression. You'll get better results, with less work, if you don't "skip a grit". Keep the grit changes to 50% or less.

If you just finished sanding with 120 grit, your next abrasive should be no higher than 180 (120 + (120 * 50%)). Continue the progression through the grits until you're happy.

I usually finish each grit sanding with the grain, not cross grain.

Burnishing can be a problem, but not usually if you're working by hand. Burnishing requires heat. If you work gently, and don't use the sandpaper like a shoe polishing rag, you should be fine.

Don't sand in the same direction ALL the time. That can force the wood fibers on top of each other and compress them. It looks like burnishing, but it's not the same (no burning). If this happens the surface won't take finish properly and your only recourse is to backup to and pretty much start over.

Stopping at 400 (without sanding marks) is fine if you're using a "filling" finish. If I'm using a oil or oil/wax finish I typically go up to 1000. You'll see a difference in the clarity of the grain, and feel the difference over time.

Just a couple of suggestion. Jump in and give it your best shot. Don't be afraid to experiment. A friend of mine used to say "it's only wood"