MDF -is- a mixed bag.  I agree with the response that it's not a good substitute for plywood.  it generates an amazing amount of dust (my brother and I made built-in bookcases with it and the garage was covered in about 1/4" of really fine dust that got into everything).  It cuts and shapes well with power tools (I think my problems with a router are due to my inexperience with that tool, rather than problems with MDF.)

However I'm using MDF for a specific project as the 'subground' for my roundhouse and turntable.  Both are mounted on a single pice of MDF to make sure the TT and roundhouse stay in alignment, including the TT's motor.  MDF is a bit easier to work for this and the fact that it comes in thicknesses to match styrofoam is a feature.   

But I think I should have primed the MDF before I glued the TT pit, and I'll go back and prime it (painting carefully around that turntable pit) before I do much else.  What I should have done, with 20-20 hindsight, is used some of my cabinet grade baltic birch plywood for this.  But what's done is done :-)

The other problem with MDF is the health concerns about outgassing formaldehyde.  You can get 'green MDF' that is supposed to not have these harmful chemicals in it.  

dave

Folks often suggest using MDF instead of plywood.  It's heavy, very heavy.  The best solution is AC plywood, one side good, one side okay in 3/4" 5-7 plies. CDX 3/4 is cheaper and ugly, you may have voids. Check out the comparison chart here: http://www.hstech.org/howto/material/plywood.htm  then decide.

Bernie Halloran

NYK&W

Another grade of plywood that is great for model Railroading is BC grade it has everything AC has except the Best side has little football shaped plugs but is just as smooth as the AC on the top surface just not as pretty. AC is designed for exposed projects like work shop grade cabinets where the A side is the exposed side and can be stained where BC is considered Paint grade where you can build the same cabinets but you wouldn't stain the finished project due to the football plugs where Knots were cut out and its about 25% cheaper than AC and about 20% more expensive that CDX which is construction grade Exterior plywood(Uses Water proof Glue to assemble the layers) which has the layers including the top surface full of ugly knot holes with pieces of the knots missing through all the layers.

Dan

Hmm, I'm gonna have to disagree. I've used 3/4" MDF for the benchwork for my last 3 layouts and I find it quite a bit better to use than plywood. Since my prior job was building and installing subwoofer speaker boxes in cars, I have had plenty of experience working with the stuff. It does make a bit of dust, but not as much as Homasote. And you don't have small chips flying everywhere. Plus there is never a worry about getting splinters in your fingers. Yes, it is heavy as a full sheet, but when cut up into 2.5" strips (like I did for my benchwork), the weight argument kinda goes out the door, since ripping it to around that size makes it easy to handle and move around. Now if we are talking about using it for a portable layout, then I would not recommend it. 
The biggest reason I use it is for sound (or lack of). It has excellent sound deadening qualities, that is why MDF is used for audio speaker boxes and not plywood. Pricewise, it's cheaper for a sheet of MDF than Plywood around my area. And a sheet of MDF is 1" bigger both ways, so that also helps when ripping it into strips. The issue of sawblade dulling would be true if you are cutting ALOT of it, way more than you would be for a home layout. I've had the same blade in the table saw for my last 3 layouts and a few subwoofer boxes here and there. It still cuts just fine.  As to the comment of waterlogging, I would say yes, if you are running a hose on it. For our scenery applications (including ballasting and plaster), I have had no problems with any sort of warpage.
So for me, I wouldn't think of using anything else but MDF for layout benchwork.

Mike

www.owensvalleysub.com

I have to agree with what Mike says about MDF, right up until I have to use fasteners with it.  I wonder about the humidity in some areas being an issue but under "normal" conditions it seems to be fine.

That said, will I be using MDF for benchwork.  Probably not.  I just like working with dimensional lumber and plywood better and sheets of MDF are too heavy as noted.

My son has been in house construction and they use MDF for everything. He did some shelf construction for us, putting extra shelves in all our closets using MDF.

Many of the shelves are now sagging. They're in closets and they're still quite functional, but most of them now have very visible sag. Plywood would not sag like that.

MDF has its place in construction, but if sagging would create problems, it's best to use something like plywood or dimensional lumber that's less suseptible to sagging over time. Or plan on putting in lots of extra legs and bracing, which starts to defeat any cost savings.

Begging your pardon Joe, but I've made shelves with plywood which over time proceeded to sag. They were about 30" wide and supported at the ends. I ended up rabbiting a piece of 1x2 stock and gluing it to the front of the shelves to give 'em some backbone.

That said, I've never noticed and sag in plywood subroadbed that was supported at 16" intervals (or perhaps the sags were small enough I couldn't see 'em)

Perhaps using hi quality plywood like baltic birch would make a difference but that stuff is really expensive.

FWIW

Charlie

Okay, you can make anything sag if you use thin enough stock, run it a sufficent distance without support, or put enough weight on it.

I just know this 3/4" MDF sags very visibly - I've never seen plywood sag so obviously unless is was really thin stock - like 1/2" or less. Plywood of 3/4" or thicker with supports every 24" or less with normal loads on it doesn't sag like this!

I met the folks from MartinJohnson Mfg. at the National Train Show in Anaheim in July of 2008.  Their modular benchwork looked like it would be ideal for me to use for a 7 foot by 9 foot "L" shaped switching layout built on top of book shelves in a spare bed room.  The standard module is 2 feet x 4 feet, but getting 2 made up at 2 feet x 5 feet only involved a small extra charge and since I could order the modules without legs, I was able to save a bit more.  The room I was going to use is now a play room for my grand kids, and I've been busy finishing up a house remodel, so the railroad is still waiting to be started.  I've been concerned that normal scenery construction would cause the mdf that is used for the table tops to warp and swell, but it would appear that it needs a lot of soaking before the mdf will swell from what I've read here.  Is that correct, or do I need to seal the mdf before I start to add scenery?

Now I realize that the following application is a heck of a lot wetter than any of us expect our layouts to get... but it's my one direct experience with MDF -

I had several chunks of the stuff that were left over when my brother-in-law bought a sheet of it to use for making some furniture-like something. When my wife and I bought our house the mailbox was pretty thrashed, so I bought a new one. In order to mount it on the steel post and beam that someone put in for the neighborhood, I had to put something screwable/nailable underneath it - so one of the chunks of MDF went there. That was nearly 4 years ago... and right now I'm wondering what I'm going to replace it with since it's almost too soggy to hold screws anymore.

So it does soak up water if exposed to it. And it's heavy.... I'm reminded of what a friend of mine once said about particle board: "Twice the weight at half the strength."

I think I would just remove the mdf and replace it with a piece of dimensional lumber cut to size, painted, and sealed before installation.