Using MDF for light-duty furniture to be painted

[John Pennisi]

I'm planning to use MDF as the primary material (I know--horrible dust, very little structural integrity) for a pair of end tables and my primary question is over how long (or short) a length of 14" width, 3/4" thick MDF will the MDF stay 100% rigid with no sag. Thoughts?

[Mike Heidrick]

For a small project MDF might be more expensive by the time you prime it enough to paint it. Go harvest some free oak pallets if you have too.

[Jim Neeley]

John,

There is no wood that will take a load without some sag since it's the force opposing the "bend" that holds up the load; it's just a matter of how much!

You can get a feel for it by using a shelf sag calculator; try googling "sagulator". It's written for wood but MDF is available near the bottom of the drop-down list.

Jim

[Julian Tracy]

MDF will work great for a couple of small end tables. Beauty of it is it's perfectly smooth - I'm not a big fan of Oak grain painted...

Only area that'll require additoonal attention is the cut and/or routered edges: they'll soak up the paint and get fuzzy. The solution for that issue is to treat them with a water glue mixture a couple times and then sand well before you apply the primer. That'll seal up and prevent the fuzziness and give you a crisp shiny painted edge. The flat parts won't soak up paint at all - paint then just like you do anything else.

Real question is how you plan on doing the joinery...

JT

[Jim Neeley]

"Real question is how you plan on doing the joinery... "

+1!! MDF splits mighty easily.

It as absorbs water so be sure to shield it.

[Todd Burch]

"Over 14" in length" - does that mean your overhang will be 14" in length or the entire length of the top is 14"?

You are looking for 100% rigid with no sag. Over what time frame and under what load? And, how is the MDF fixed prior to the 14" and how big is the rest of the piece? What humidity levels will it see?

Not enough info to answer the question.

[Michael W. Clark]

My opinion is that MDF is fine for painted furniture. Like others have said, treat the edges to get an even paint finish. You can prime the whole thing with Kilz and that helps even it out as well. As far as the sag, it depends on the weight applied and span. It doesn't sound like there is going to be a lot of weight being an end table and only 14" wide. If it warps over several years, probably not a big deal. Its not like this is a family heirloom to be handed down for generations.

Another step up in paint grade rigidity would be to use plywood and a secondary wood like poplar. With a plywood or MDF top, you could pocket screw it or glue it to the stretchers. You could also screw it from the top and patch the holes with wood putty since you are painting it. MDF legs would require a little more thought and knowing your arrangement, I would probably opt for solid wood legs or plywood glue-ups and cover the edges.

Mike

[Larry Browning]

MDF will work great for a couple of small end tables. Beauty of it is it's perfectly smooth - I'm not a big fan of Oak grain painted...

Only area that'll require additoonal attention is the cut and/or routered edges: they'll soak up the paint and get fuzzy. The solution for that issue is to treat them with a water glue mixture a couple times and then sand well before you apply the primer. That'll seal up and prevent the fuzziness and give you a crisp shiny painted edge. The flat parts won't soak up paint at all - paint then just like you do anything else.

Real question is how you plan on doing the joinery...

JT
Drywall mud works well for treating the edges as well.

[John Pennisi]

Thanks for all replies so far; they are much appreciated.

The primary reason I am going w MDF is because the client wants a flawless black lacquer gloss finish that I will be spraying on. The hints on how to treat edges prior to this will be very helpful..

Also I don't see it needing to bear more than 15-20 lbs max but I still worry about the structural integrity of the mdf top w quite a bit of overhang.. About 18" worth w a 3/4" MDF top. I am considering routing out a section underneath the top and laminating a piece of 1/2" baltic ply to add rigidity but I am not sure if it is worth the work. It would probably be effective and easily hidden on the underside of the top, yet the edges will likely be tricky to adequately prime to take any of the gloss finish..

John

[John Pennisi]

Joinery will be lined biscuits or dominoes.

[George Gyulatyan]

For the finish, take a look at Target Coatings EM6500 base coat which you can have tinted followed by one of their non-yellowing top coats for a great glossy finish.

I've also used General Finishes milk paints followed by their Polyacrylic with a good result. I did that on plywood. I started with several coats of dewaxed shellac, paying close attention to the edges, where I basically kept sealing them with shellac and sanding, until I didn't see any of the layers, then followed that by 3 coats of Milk Paint, sanding inbetween the layers, which further reduced the appearence of grain, and then topcoated with 4 layers of Polyacrylic again sanding between each coat. Gave me a great deep black glossy look.

You should be able to use shellac as sealer/primer on MDF, which would probably give you smoother base to work off of than regular paint primer.

[Todd Burch]

Go to pretty much any furniture store and look at their fancy dining room tables. You'll see 18" of overhang of 3/4" MDF (with a paper thin veneer on the top) all day long with no additional supports. How can you tell? Look underneath! Unsealed and unfinished.

[Mel Fulks]

Reinforcement is probably a good idea. Instead of routing and filling I'd use real wood with edges beveled back several inches and then screwed under with the thin edge set back three inches.That type of support was sometimes used under fine Federal style tables with thin tops designed for grace. Such an addition is seldom noticed and painted black will not be seen.

[Michael W. Clark]

John,
If you could post a sketch of the table, commentary on rigidity and finishing would be more pointed to your application. 18" of overhang may not be a problem, but if your base is only 14" square and you have 18" overhang all around, it could get unstable. Sorry, I may misunderstand, just trying to tie together the 14" dimension you gave initially and the 18" dimension in your other post.

Mike

[Harvey Melvin Richards]

I have finished MDF with a gloss black. It's difficult. You want very good light to start with. Bondo and spot filler from an auto body supply house will be your friends. Use a sandable primer and sand and fill the surface and edges until there are no voids. It will take multiple coats and sanding in between to get the finish correct.

If your thickness can handle it, make a torsion box for the assembly. It will be very rigid that way.

[John Pennisi]

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John,
If you could post a sketch of the table, commentary on rigidity and finishing would be more pointed to your application. 18" of overhang may not be a problem, but if your base is only 14" square and you have 18" overhang all around, it could get unstable. Sorry, I may misunderstand, just trying to tie together the 14" dimension you gave initially and the 18" dimension in your other post.

Mike

[John Pennisi]

The one thing that those images don't show is that the vertical supports on either side of the empty space (not the thin supports that are flipped in, but the two sections that provide most of the vertical support) are actually 1 1/2" thick (not 3/4") so as to serve as more of an anchor for the top and bottom.

[Todd Burch]

Good luck!

[John Pennisi]

Good luck!

Thanks? Anything else Todd? Trace of sarcasm there??

[Todd Burch]

I think it will fail miserably, but I wanted to be positive and supportive!

[John Pennisi]

I disagree.. And thanks!

[John Pennisi]

Todd, feel free to elaborate a bit if you have a moment.. Thank you.

[John Pennisi]

I think it will fail miserably, but I wanted to be positive and supportive!

Ok Todd feel free to elaborate or suggest. Thanks.

[Myk Rian]

For a table like that, MDF is a bad choice. Real wood is the way to go.
MDF will not be able to hold up to the forces of anything set on the table.
Trust me on this. Todd is completely correct. You have to consider mechanical loading, and the ability of MDF to withstand it.
The first table is one my MIL had. The second is one I made out of cherry.

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[Harvey Melvin Richards]

MDF is great for a lot of things, that table isn't one of them.

[John Pennisi]

Got it loud and clear, this is why I was asking about possibly laminating baltic ply to mdf at the start. Any recommendations in what wood might take a black gloss lacquer the best?

[James Baker SD]

What are pianos made from? They certainly get a high gloss on most of the ones I've seen.

[Myk Rian]

Here are some tips from woodweb.
http://www.woodweb.com/knowledge_base/Duplicating_a_black_lacquer_finish.html

[George Gyulatyan]

Agree with the rest. That particular design in MDF would spell disaster. Now, if all parts were build as torsion box, that might be a different story, but if you want to keep it to 3/4" then you want to go either solid wood or baltic birch/Europly/ApplePly.

[Bryan Slimp]

I can't see a way that those tables could be made using wood.

Powder coated steel? Carbon fiber?

[Keith Hankins]

Well, IKEA has made a fortune selling junk made of it so why not right ? Seriously, I don't see it standing up. I have made a lot of jigs out of it and when making prototypes of pieces it's great and cheap. It could be sturdy if you douple up and engineer the heck out of it, but then you use a lot more and the cost savings go right out the window. either get some ply, or pine, or better yet if you are going to paint, get poplar its cheap and takes great paint finish, and is strong. JM2CW.

[George Gyulatyan]

I can't necessarily buy that "it can't be done" as there are designs out there that accomplish this just fine. Someone here has even built "Z" chairs using ingenious joinery using Dominoes, so it's certainly possible. However, careful selection in materials and construction methods is in order.

The question I have is this... is 3/4" thickness cast in stone?

The reason I ask is... take a look at below (grabbed from a Google Image search):
239749

Most likely construction for this is based around a torsion box.

[George Gyulatyan]

Do a Google Image search using "cantilever modern coffee table" as the search term. You'll find a lot of designs using wood and other materials.

[Myk Rian]

I can't see a way that those tables could be made using wood.
????? I have 2 of them in the house.

[Chris Friesen]

You've got an 18" lever arm on one side, and only 3" of lever arm on the other side (the pivot point is the inside of the narrow vertical pieces). If you put 20lbs in the center of the table, you're looking at roughly 50lbs of force on the narrow vertical pieces, and about the same amount of force trying to pull the joint apart. If you put that 20lbs on the edge of the table, you now have 120lbs of force on the narrow vertical pieces and the same amount trying to pull the joint apart.

If you were to glue and screw the main joints I think you'd be okay as long as nobody tries to sit on it or put something really heavy on it.

[John Pennisi]

You've got an 18" lever arm on one side, and only 3" of lever arm on the other side (the pivot point is the inside of the narrow vertical pieces). If you put 20lbs in the center of the table, you're looking at roughly 50lbs of force on the narrow vertical pieces, and about the same amount of force trying to pull the joint apart. If you put that 20lbs on the edge of the table, you now have 120lbs of force on the narrow vertical pieces and the same amount trying to pull the joint apart.

If you were to glue and screw the main joints I think you'd be okay as long as nobody tries to sit on it or put something really heavy on it.

Chris, thanks. This is exactly the type of engineering take I was looking--and should have specifically asked--for. I assume you are making this point with the assumption that I'll be using solid wood (probably poplar). MDF is out of the question, correct? Thanks again, John

[Chris Friesen]

I assume you are making this point with the assumption that I'll be using solid wood (probably poplar). MDF is out of the question, correct? Thanks again, John

Given how cheap MDF is and how simple the design is, I would probably just make a couple prototypes (don't bother finishing them) and test them to destruction. That should give a ballpark idea as to how strong they are.

I think your best bet from a strength perspective would probably be baltic birch (or appleply) rather than solid wood. Ideally with solid wood you'd want the long grain of the cantilevered pieces projecting outwards for strength, but then you're either gluing crossgrain or else gluing onto endgrain, either of which is ideal.