An idiot and his sander...

I've already built a few simple projects, but I find that the ignorance lingers, even as regards some very basic concepts. I learned a ways back (the hard way) that "modern" ply can't be sanded a great deal without the substrate peeking through. My error stems from '70 era high-school shop training that was better suited to solid wood, and the memory of older plywood whose top veneer was more than a few molecules thick.
So my question is this: How should I sand?
I'm considering building some plywood bookcases; tall (7'?) narrow (24" x 15"?) tower-shaped things to flank a wide window. Birch, or maybe oak. I've been told here that I need not start with coarse paper on ply, but I'd like some details.
I'd be using a very old third-sheet orbital sander. It's what I've got, and it seems to work well. The weight seems to damp some of the vibration too.
http://www.flickr.com/photos/gdguarino/6410114261/lightbox /
So what grits should I use? And how long should I sand each area? This project, should it actually become reality, would require sanding the better part of three sheets of ply on both sides, not including whatever I use for the backs. I'd like to be efficient.
As an aside, what should I use for the backs, anyway? I'm sure 3/4" is not necessary, but what do you recommend?
As always, thanks in advance.
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On 12/5/2012 1:10 PM, Greg Guarino wrote:

as little and light as possible, dependent upon how you're finishing them.

a little at 220 is about what i'd do. almost anything rougher will go through the veneer too quickly

door skins

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Greg Guarino wrote:

There are two main reasons for sanding...
1. to make the surface flat
2. to make the surface smooth
Hardwood plywood is already about as flat as it will ever be; it is also generally pretty smooth. If it isn't smooth enough for you, you can make it smooth by sanding the finish, not the wood. If you just gotta sand the wood I would suggest a grit no coarser than 180. How long to sand? Until your fingers tell you is is at the smoothness that you want.
One of the better things to use if you need to sand the ply is a sanding sponge. I'm talking about the ones about 4x5x1/4. Fine or superfine grit. They are good because the sponge deforms so that slightly depressed areas get sanded without cutting down the higher ones excessively. The bad news is that you do it by hand; the good news is that it takes very little time.
For backs, 1/4 ply works just fine. So does 1/2, ditto 3/4. Be sure to build in some way of securing the cases to the wall as they *WILL* tip.
--

dadiOH
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You missed a very important reason for sanding, or planing. For finishing you want fresh wood, not wood that has oxidized. It does make a difference. The wood should not be exposed for too long, or the finish MAY be less than it could be.
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On Dec 5, 10:45 pm, tiredofspam <nospam.nospam.com> wrote:

Hmmm. I'll remember that. Thanks.
Being the finishing ignoramus that I am, I do a lot of "testing". Every time I have used any sort of finish, I put some on different kinds of scrap as well; oak, maple, ply, even a planed-down bit of 2x4 once. But I don't usually bother to sand them first. Sometimes it seems as if the finish doesn't penetrate very well.
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On Thursday, December 6, 2012 6:06:44 AM UTC-6, Greg Guarino wrote:

Many finishes don't/can't penetrate at all, but just lay on top. *I am aware that you will apply your finish coats to raw wood, hence your testing on raw wood. Just because a finish doesn't penetrate (Deeply? How deep?) doesn't mean your application has failed.
What symptoms/results are you referencing/experiencing, for your assuming the finish doesn't penetrate (isn't penetrating) very well? *I'm wondering if your term "penetrate" is the appropriate term for the results you are getting "Sometimes, it seems....".
Stains, conditioners, sealers and oil finishes may/will readily penetrate raw wood. A subsequent top coat is exactly that, i.e., lays on top. A finishing varnish, shellac, lacquer, poly coat - not proceeded by a stain, conditioner or sealer - will penetrate raw wood, to some extent.... a particular specie of wood can/does dictate how a finish (alone) is absorbed, if at all. That's one of the reasons for doing test applications, to see how the different applications (or combinations of, i.e., w/ stain, conditioner, sealer, etc.) apply.

I agree, you want fresh wood, to be finished.... or stained, conditioned, sealed, etc., also. You want clean wood, also, hence, use of a tack cloth or vacuuming may be beneficial, after sanding.
Sonny
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On 12/5/2012 2:10 PM, Greg Guarino wrote:

Assuming that the surface is in good shape and you are actually using cabinet grade plywood there is no need to start any coarser than 180 grit and I never use any thin any finer. And still, be careful.
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On Wednesday, December 5, 2012 12:10:31 PM UTC-8, Greg Guarino wrote:

I would only sand by hand with a rubber or other block and just use 220. Really just need to knock down a few fibers and equalize the surface scratching on any decent ply so any stain of finish will look consistent.
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On 12/5/2012 2:10 PM, Greg Guarino wrote:

For what you describe above, minimum of 1/2" plywood, with the back preferably dadoed into the end panels, top and floor; and with fasteners through the back into stationary shelving, as below.
Face frame, or frameless?
If face frame, at least one built in, stationary shelf that spans the inside width, preferably dadoed/glued into the end panels. The rest of the shelves can be adjustable.
If frameless, consider using 3/4" ply for the back, with two built in, stationary shelves that span the inside width, preferably dadoed/glued into both the back and the end panels. The rest of the shelves can be adjustable.
For any standalone piece that tall you will need to either attach them to the wall in some manner, or you will need an "anti-tip" device of some sort.
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I've gotten an interesting range of answers so far, but all seem to recommend even less sanding than I had imagined. Point taken. Should save time too, as so often happens when you do it right.

Face frame, I think. For looks, mostly, but maybe it will stiffen the shelves a tad also.

I find that I practically never actually "adjust" adjustable shelving. I plan to take stock of what the cabinets will hold (books) and make all of the shelves fixed.

I plan to attach them to the wall. What other option would there be?
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