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I am starting to plan some built-in bookshelves that will be painted with mat or eggshell paint.

The fronts of the shelves and upright will be moulded maybe with a router or softwood mouldings glued on. We have not yet decided if the shelves will be fixed, or adjustable using an inset bookcase strip.

The two areas of concern I have are:

  • I don’t wish the shelves to slag under the weight of the books.
  • I want a good paint finish.

MDF gives me the good finish and the option to router edges; however is it stiff enough for book shelves?

Chipboard is a cheap option, but can I router slots in it for the bookcase strip?

Is it possible to get a good paint finish on plywood without spending a long time sanding it?

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If you don't want to spend lots of time sanding the plywood, you can just buy a more expensive grade ... but if you're painting it, I wouldn't bother. –  Joe Oct 16 '12 at 16:43

2 Answers 2

up vote 4 down vote accepted

I would use MDF. It is uniform, takes paint well and doesn't warp. It also routes well.

It should hold the weight I'd expect in a bookshelf (supported at both sides, about 30" wide and no deeper than 18")

If you're really concerned with the sagging you might add a single center support bracket.

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30cm is not very wide for a bookshelf, do you use 18mm mdf? –  Walker Oct 16 '12 at 15:37
    
@walker: he's probably using inches, so multiply by 2.5ish: 75cm wide by 45cm deep. –  Niall C. Oct 16 '12 at 15:47
    
+1 for MDF. I don't know about ultimate strength or long term creep, but for the same thickness, MDF is stiffer than the other options. Fixed shelves fully supported on 3 sides will be significantly stronger than only supported on 2 ends. A substantial front moulding detail will also significantly stiffen the shelf. –  bcworkz Oct 16 '12 at 19:11
    
MDF, only way to go for a paint finish –  shirlock homes Oct 16 '12 at 21:37
1  
MDF is great stuff, but keep in mind that it paints differently on cut edges than on the factory faces. It can be very thirsty on the edges, so your routed edge detail may not paint up quite the same as the rest of the shelf. Multiple coats and sanding may help, otherwise cap the ends with softwood as you suggested. –  mac Oct 17 '12 at 19:02

How much the shelves will sag depends not only on the materials you use, but also whether or not the shelves are fixed, whether there's a hardwood strip along the length, and how they are attached. The magic number to watch out for is 1/32" (about 0.8 mm) of sag per foot.

MDF is incredibly heavy - your bookshelves will weigh a ton if you construct them entirely from it. Further, they will sag under their own weight if the span is wide enough, so be cautious if you intend to make the shelves wide without additional support. You might consider attaching hardwood edge strips or building a small torsion box frame beneath each shelf if you intend to have wide spans. This, of course, is bulky and unsightly.

That said, MDF does rout and paint incredibly well if that's more important to you and your design doesn't call for wide, unsupported expanses. However, tight-grained, close-pored hardwoods also paint well. If you can find Poplar or Maple veneered plywood (all of my local home centers carry both), that should work pretty well. You can also fill the pores of open-pored woods to make them accept paint better.

As for the edges of plywood, a common technique is to attach a hardwood strip because it allows for routing without all the ply layers showing through, and it adds quite a bit of load bearing capacity. Also, you shouldn't have problems with plywood warping if it's kept in a relatively dry area, especially in a climate-controlled area like inside your house. And if you go all out and get high-quality plywood from a lumber yard rather than a home center, it's often even more stable due to a greater number of plies and less voids/gaps.

I wouldn't even go near chipboard. I'll admit a little bias because all of the manufactured furniture I've ever owned made from chipboard either sagged, chipped, or buckled under stress (in a matter of 2-3 years, in most cases).

Check out http://www.woodbin.com/calcs/sagulator.htm if you get a chance. You can play with various materials, edging strips, whether or not the shelf is fixed or floating, and so on. It also has great tips to watch out for.

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Yes, plywood trimmed with hardwood is great for shelving. +1 –  fungku Apr 7 '13 at 16:33
    
The hardwood on the edge also makes the shelf look beefier. –  Eric Gunnerson Apr 8 '13 at 2:13

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