Treating MDF

[mark]

I'm about to start a project which basically involves building lots of MDF boxes then stacking them in a brick-like fashion to make a wall-to-wall boo kshelf.

The boxes will be constructed out of 18mm MDF, and be either 280x350x400mm or 180x250x400mm in size. (I'll use 2mm MDF for backs.)

I haven't decided yet whether to use veneered MDF. If I don't then I need t o treat the whole lot (probably a satin varnish, although I may paint them as MDF is quite boring), but if I use veneered then I only need to treat th e cut edges, none of which will be visible once complete (assuming I use a veneer edging strip on the visible edges). I can't get 2mm veneer for the b acks so I'll paint those to roughly match whichever veneer I choose.

What's the best way to treat the "invisible" edges? I'm thinking both in te rms of the product and the method - I obviously don't want the product comi ng over the edges onto the veneer, and my DIY skills are "basic". I conside red stacking all the parts together and treating all the edges in one go bu t I wasn't sure if I'd end up sticking them together or leaving a rough edg e when they were separated.

A couple of people have suggested diluted PVA to seal the edges, which woul d certainly be cheap but would it work? (The shelves will be going into a b edroom so I don't expect them to be damp.)

In total I'll have around 60 boxes to build so anything complicated is out, I don't have the patience!

[Tim Watts]

On Tuesday 12 February 2013 08:51 [e-mail] wrote in uk.d-i- y:

I would not - the water content will swell the MDF.

Perhaps a diluted polyurethane varnish, spirit based?

[stuart noble]

I've done a bit of this for my siblings over the years, mainly 15mm, and the various finishes they were threatening to use never materialised. That neutral, scruffy mdf look grows on you. Any clear finish tends to give them a dark tan "wet" look, which is not very appealing on mdf. Try it on a scrap first. Sixty boxes? Blimey, you're going to waste a lot of space with the thickness of the mdf. I prefer to make half a dozen modules with shelf supports so that you can change the spacing. There always seems to be one thing that's slightly too tall for any of the boxes. I'm sure you have a plan for the assembly but FWIW I'd have the whole lot cut to size by a timber merchant, and treat yourself to an electric stapler. Great for putting the backs on and holding the four sides in position if you're working alone

[mark]

I have some MDF shelves that were varnished (to protect the MDF not for eff ect) and as you say, the effect grows on you. But that was stuck in an offi ce, this is going to be a "feature" in a spare bedroom that is supposed to become more of a library/snug, so I want to at least try to make something "nice".

Using veneered MDF is obviously a lot more expensive, but is somewhat offse t by the saving in varnish and would be a considerable time saving, for a m uch better end result, which is why I'm leaning that way at the moment.

The bottom boxes (35cm internal height) are tall enough for box file (I hav en't 100% decided on dimensions yet). They will likely be filled with magaz ine archives - ie will be heavy. I need to be sure they will take the weigh t without bowing. (They'll also have the weight of the upper boxes to suppo rt.)

If I could be convinced that a thinner MDF would suffice I'd drop down a si ze - with 18mm I'm playing it safe.

By constructing it from boxes laid out like bricks (ie each row is 50% offs et from the ones either side of it) I end up with quite short gaps between verticals without making the shelves too short, which should lend strength. However this is all speculation from a software engineer with no experienc e of constructing projects like this so if anyone thinks I'm taking a big r isk let me know!

Absolutely! In fact it was partly my lack of skill when it comes to cutting timber that lead to the design - this is very easy to get supplied almost in kit form just from a set of measurements. Aside from treating the MDF, a ll I should need to do is to put in 2 or 3 screws on each edge and either p in or staple the backs on. Given that I have a lot of nearly identical boxe s to make, I may even make up a jig to hold the sides in place while I scre w them together.

I wondered about this too. It wouldn't be worth me spending a lot on one th ough as this could well be the only time it gets used. I can get a cheap on e for about £20 with 1000 staples and 1000 nails, or a Stanley one for tw ice that - is it worth it?

[stuart noble]

Most kitchen unit shelves are 15mm chip, and an unsupported span of a metre is pretty normal. Mine are stacked with crockery, which is a hell of a lot heavier than books.

Clamping a piece of the shelving to the sides gives you a "ledge" to sit the cross piece on. If screwing thorough the sides into the edges, these might be useful

but I've found drywall screws just as good. Standard woodscrews tend to be a bit too fat, and will swell the mdf if not 100% accurately positioned. Oh, and get one of those combination bits that do the hole and the countersink in one. Then, with a jig, you can pre-drill all the sides accurately in one operation. You don't want to be doing that by eye!

I've had a Bosch for years and wouldn't be without it. Takes generic staples too, so you're not tied in to expensive Bosch stuff. IME forget the nail option on cheap tools, they misfire so often it'll drive you crazy.

[Geoff Pearson]

I have bookshelves in 18mm mdf - they have bowed under the weight at 750mm length. MDF is not as rigid as it looks. Pine is stiffer - shelves that long stay straight.

[harry]

F boxes then stacking them in a brick-like fashion to make a wall-to-wall b ookshelf.

m or 180x250x400mm in size. (I'll use 2mm MDF for backs.)

to treat the whole lot (probably a satin varnish, although I may paint the m as MDF is quite boring), but if I use veneered then I only need to treat the cut edges, none of which will be visible once complete (assuming I use a veneer edging strip on the visible edges). I can't get 2mm veneer for the backs so I'll paint those to roughly match whichever veneer I choose.

terms of the product and the method - I obviously don't want the product co ming over the edges onto the veneer, and my DIY skills are "basic". I consi dered stacking all the parts together and treating all the edges in one go but I wasn't sure if I'd end up sticking them together or leaving a rough e dge when they were separated.

uld certainly be cheap but would it work? (The shelves will be going into a bedroom so I don't expect them to be damp.)

t, I don't have the patience!

There is special primer paint for MDF. Other paints allow pinholes to form as they dry.

[fred]

A 1 metre span of 15mm chipboard will carry very little unless pinned along the back to the back panel and even then I would't risk a lot on it. We us e 25mm for shelves in an office environment and don't go beyond 900mm with it. Even at that it will still deform slighty under a load of books.

I would not be a big fan of screwing into mdf. If necessary we use drywall screws but even with pro drilling etc. it can still split.

F.Y.I. and further enlightenment read here

[Mark Rogers]

That ties in with what I figured, hence using 18mm MDF doubled up. But I th ink I may still be overdoing it - the box width is 40cm but by staggering t hem it is supported underneath at the midpoint, but then it is also taking weight from above at its own midpoint which works against it....

Even so, 18mm doubled is 36mm (1.5"). 12mm doubled to give me 24mm (1") wou ld surely be enough, wouldn't it? (it's a big project to get wrong!)

That is another advantage of the thicker MDF - I'm going to be quite a long way from the edges. This could be where I'd regret having 12mm.

Just so we're clear, the design is something like this:

---------------------------------------------- | | | | | | | | | ---------------------------------------------- | | | | | | | | | ---------------------------------------------- | | | | | | | | | ---------------------------------------------- | | | | | | | | | ----------------------------------------------

.. with the span between uprights 40cm, and because each box is independent ly constructed from 18mm MDF each horizontal and vertical is actually two a djacent 18mm pieces, ie 36mm total)

[stuart noble]

As long as the bottoms rest on the floor, you'll be ok

It's 15mm you need. You can screw into the edge, but it doesn't look ungainly.

I think you could store car engines on that :-)

[polygonum]

Certainly seen that claimed several times.

But when I painted lots of MDF shelves, I used ordinary emulsion - rollered on - two or three coats. And a topcoat of quite a good water-based acrylic floor "varnish" (got from Screwfix - made somewhere in Essex, IIRC).

Excellent, remarkably tough finish - though had to dispose of some recently and they looked immaculate as they were turfed into skip at dump.

[Chris Holford]

In article , Tim Watts writes

snip

snip

What about clear cellulose dope?

[Andy Champ]

My bookshelves are 50 and 60cm supports with 15mm chip with a laminate coating. They don't bend enough to notice.

That design seems to have the uprights staggered. I think it would be better if they went all the way to the floor, else the load from the upper shelf is carried by the shelf below and not transferred through an upright.

Andy

[Mark Rogers]

Thanks, I'll get the timber merchant to price that.

It also crossed my mind that the smaller boxes at the top may not need to be the same thickness but I'd need to mock something up on my PC to see what it would look like. (Any recommendations for a sort of CAD "Lite" for this sort of job much appreciated.)

I did say it might be overkill!

[Mark Rogers]

Structurally I can't decide whether you're right or not - I'm 50% on the si de of staggered being better, and 50% on the side of it being worse. But I prefer the stacked brick look so I guess my goal would be to find a way to make it strong enough, rather than change it to use thinner materials. With in reason, of-course - if the design is simply not workable I need to go ba ck to the drawing board.

[polygonum]

Many years ago, I did something not a million miles away. Used 9mm plywood (IIRC). Each box was stuck together with glue and some fine nails (can't remember exactly which sort - maybe panel pins).

Used them as book (and everything else) shelves. Strength and rigidity were not an issue though maybe one or two had the slightest hint of a bow when very heavily loaded.

Always believed that the back being made of the same ply made a big difference to stability and rigidity. It isn't there just to keep dust and spiders out. I would not wish to rely on thin MDF to stop the whole structure distorting and even collapsing. So either make backs out of similar or add some form or diagonal bracing.

Reason not to use ply is the difficulty of getting all the edges finished nicely. The edges always looked crap but the effort of putting some extra lip of wood was too great.

Use suitable respirator when in presence of significant quantities of MDF dust.

[Peter Parry]

with 18mm I'm playing it safe.

You might find the Sagulator

useful.

[Mark Rogers]

Thanks, I'd used that before ages ago but forgotten where to find it.

I've now pretty much convinced myself, based on advice here, to go with ply instead of MDF. Birch ply looks pretty good (better than MDF anyway!) and should be stronger. Using the Sagulator, 500kg/ft on my 40cm spans, even at 12mm (not 12mm doubled up), produces only 0.01mm sag/ft, so I think that e ven for the biggest boxes 12mm ply should do what I need? (Again, remember that's 12mm for the box construction so the actual shelf thickness would be twice that.)

Also, I assume that ply would not need to be treated for preservation purpo ses, so treatment becomes just a matter of aesthetics? And that it should b e easier to (eg) varnish than MDF?

The only big question, if the above assumptions are correct, is how to secu re the corner joints - I assume that screwing into the ends of the ply isn' t a great idea? Would long(ish) nails be better?

As far as I can see, 12mm birch ply will cost me about the same as 12mm ven eered MDF, and give me a "real" wood finish, extra strength, and easier tre atment into the bargain, so it feels like a no brainer.... (Someone is abou t to tell me I'm wrong!)

[fred]

Ply sounds good and will look a lot better.

I know it may not be the effect you want but I would be inclined to run hor izontals the full width, or as wide as possible, and insert verticals in th e appropriate places. Board will carry a heavier load when the ends are can tilevered as opposed to being held by the ends only. As you are staggering the uprights it would be a simple matter to screw through them into the ver ticals.

[Mark Rogers]

Thanks

I haven't ruled this out. The effect can be gained anyway by still using tw o 12mm lengths on top of each other, with small cuts on the outer edge wher e the verticals are (just 1mm into the wood would give the effect of it bei ng a separate piece).

Alternatively I could switch to 25mm ply, without doubling anything up, and look at the ply as being the "mortar" between the brick-shaped "holes" - I need to try and mock this up.

The only major question I have left is how to do the joins. I've just inves ted in a basic 18ga nail gun (15-35mm nails or 15-30mm staples), which I th ink would probably be better than screws? Almost all the stresses should be pushing downwards such that everything would probably be fairly stable wit hout any screws or nails anyway (albeit very hard to construct in the first place), and any sheer there is should be countered by having backs on the units?