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I am new to woodworking. I recently completely my first job (a small bookshelf/toybox). After it was all put together, I spent a while sanding it, although it was difficult to get into the corners, and it ended up not very smooth. Since I was going to paint it, I figured that would help smooth it out - but not as much as I wanted, so I ended up sanding more and then repainting.

It seems intuitive that sanding something before assembling it would be considerably easier, yet, all the instructions I see for woodworking have sanding as the second last step (before finishing). Why would I not want to "presand" all the components, when they are more easily accessible?

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Prefinishing (and thus presanding) are talked about fairly often as a suggested step to avoid having to apply finish or sand into corners and other tricky areas.

General idea is to get everything ready for final assembly and then sand/finish before assembly. (Making sure you don't get finish on areas that need glue). Masking tape for protecting areas is a good idea.

I think there's probably not a huge deal of talk about it because it's pretty elementary stuff and an obvious good idea that most people stumble across themselves after a few frustrating experiences trying to sand into corners. Also, assuming you are using cleanly milled lumber, there should be minimal sanding required in most cases so probably a lot of articles gloss over it.

That said, here's an article by Marc Spagnoulo and a FWW podcast with a bit more detail on it:

http://www.finewoodworking.com/2016/09/30/stl-121-finishing-tips-favorite-techniques

http://www.thewoodwhisperer.com/articles/pre-finish-strategy/

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Very related:
At what stage of basic furniture making should I "finish" the wood?
Should I finish before, during, or after assembly?
The second thread was marked as a duplicate of the one above it but its Answers give much further info so well worth reading both.

It seems intuitive that sanding something before assembling it would be considerably easier

Yes, often it is much easier to smooth wood before assembly. Ditto finish it as you can see from the above threads.

yet, all the instructions I see for woodworking have sanding as the second last step (before finishing).

This could be partly due to bad instructions as you're thinking. Sadly there are loads of poor or outright bad how-tos online on any subject you care to name.

It could though be partly a case of extrapolating from specific projects and thinking it applies more widely, as actually it's not uncommon to pre-sand certain pieces or some surfaces of individual component prior to using them or installing them, precisely because it is hard (or impossible) to do it after assembly.

As wood is very frequently smoothed and made ready for finish by sanding these days boards prepared for shelving for example are usually sanded prior to assembly, possibly leaving a little touch-up sanding to the end after assembly but sometimes not even that (it is better not to have to if you can manage it).

Point of note: you don't have to sand to complete the preparation of wood. Often it can be prepared for finish by hand planing or by scraping (usually in less time and always with less dust produced). See What are the differences between sanding and scraping? for more.

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