Joining two MDF slabs, face to face, and getting it flat

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Forum topic by Mark Colan posted 09-01-2010 03:09 PM 26140 views 0 times favorited 18 replies Add to Favorites Watch
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Mark Colan

211 posts in 3046 days

09-01-2010 03:09 PM

Topic tags/keywords: question resource

I want a 1-1/2” thick MDF table top as an extension table + router table combo for my table saw. The thickness is required for a stable router table surface. A key goal is to have this table be very flat.

In one of Bill Hylton’s router books, he rolls yellow glue evenly over a surface, maybe both surfaces(?), puts them together, and clamps with cauls. He says I have to have a truly flat surface to get a truly flat surface.

I don’t have a known flat surface as large as these table tops (which will be about 27×35” before edging). I also don’t have cauls, and I don’t want to take the time and money to buy a plane, get it all tuned up, learn to use it well, and build two cauls. Maybe there is an easier way to make cauls than the planing method?

I DO have a mobile workbench (check out the Ultimate Tool Stand) which I believe to be reasonably flat, because it is built on a torsion box; it is 24×72”. It is ALMOST wide enough.

I wonder if contact cement might dry thinner than yellow glue, therefore not adding variable thickness in the dried glue. Is it strong enough? (It doesn’t matter that I cannot move around the pieces once the faces are: I intend to cut oversize than trim to size.)

MDF is very rigid. I also wonder whether flatness is simply not an issue with slabs this size.

What’s the best I can do with what I have?

-- Mark, hack amateur woodworker, Medford (greater Boston) MA

18 replies so far

View PurpLev's profile


8547 posts in 3849 days

#1 posted 09-01-2010 03:39 PM

1. don’t use contact cement – it doesn’t have a rigid glue line but a flexible one. use yellow glue. spread on both sides – since it’s MDF, it’ll soak it in a little which is a good thing and will give you a good bond.

2. making cauls is easy – bottom line – you just need it to slope from the center to the sides, so that when the sides are clamped, it’ll bend and create pressure along the center as well.

3. I think your tool stand station will be more than adequate to hold the 2 sheets during glue up.

4. little trick – if you don’t want the 2 panels moving during glue up (glue can sometime be slippery and shift the panels sideways) rub some sand paper over the panel with the glue (just a little bit) the glass remnants will create enough friction to hold the panels together

and finally – if you need hand planes, cauls, or the likes – feel free to stop by ;)

-- ㊍ When in doubt - There is no doubt - Go the safer route.

View John Steffen's profile

John Steffen

218 posts in 3256 days

#2 posted 09-01-2010 04:26 PM

Not sure if it’s helpful for you or not, but thewoodwhisperer has a good segment where he builds a flat surface out of some sawhorses and 2×4s to be used to build an assembly table top.

-- Big John's Woodshed - Farmington, IL

View Bob #2's profile

Bob #2

3810 posts in 4222 days

#3 posted 09-01-2010 04:32 PM

Your table saw top is probably the closest to flat in your shop.
I’d spray it(saw top) down with a metal lubricant or WD40 and cover it with brown paper.
Then just clamp your two pieces to the top. You can drive a couple of screws to hold them together where you intend to make the hole for your router. It then just a matter of using a toss away paint roller to coat the surfaces and then set your holding screws and clamps/

-- A mind, like a home, is furnished by its owner

View Rick  Dennington's profile

Rick Dennington

6301 posts in 3395 days

#4 posted 09-01-2010 09:02 PM

Greetings Mark,

No worries, Mark…....just glue them and screw them together….just don’t put any screws where the

router plate is going to be (but I bet you already knew that,lol). MDF is good and flat right out of the gate.

I use 1” sheetrock screws for 2 pieces of MDF…that’s plenty…..The screws act as clamps, so go for it….lol.

-- " At my age, happy hour is a crap and a nap".....!!

View Mark Colan's profile

Mark Colan

211 posts in 3046 days

#5 posted 09-01-2010 09:27 PM

@Bob2: yes, I’ll probably use my table saw top for the flat surface, good idea.

@Rick: that’s an obvious solution, and somehow I missed thinking of it.

@John: thanks for the link, I’ll look. Farmington, eh? I’m originally from Peoria. Perhaps I’ll look you up next time I’m home visiting my ma.

But PurpLev, who lives <3>ll show me how to make cauls. I will probably go that route for the learning experience.

-- Mark, hack amateur woodworker, Medford (greater Boston) MA

View richgreer's profile


4541 posts in 3275 days

#6 posted 09-01-2010 09:57 PM

In my opinion, the best way to glue up 2 pieces of MDF is with a vacuum press. Of course, many people do not have access to a vacuum press. Nonetheless, if you have access to a vacuum press that is large enough, that is your best option.

MDF is naturally very flat material. I don’t see a vacuum press introducing any non-flatness into the MDF.

-- Rich, Cedar Rapids, IA - I'm a woodworker. I don't create beauty, I reveal it.

View Mark Colan's profile

Mark Colan

211 posts in 3046 days

#7 posted 09-01-2010 09:59 PM

@RG: that does sound like the best approach. I’m building a local network, so maybe with time I’ll find people who can provide access to tools like that.

-- Mark, hack amateur woodworker, Medford (greater Boston) MA

View PurpLev's profile


8547 posts in 3849 days

#8 posted 09-01-2010 10:03 PM

actually, reading through the comments – I think the screwing method would be the easiest, and fastest. esp. since the underside of the table is not visible, so the screw holes won’t not be seen. this method will ensure equal pressure along the entire surface, and is easy to accomplish.

we can still make cauls though ;)

-- ㊍ When in doubt - There is no doubt - Go the safer route.

View phlepper's profile


21 posts in 3453 days

#9 posted 09-02-2010 12:50 AM


I created my workbench top the same way (actually have done it four times now for different benches). I just pre-drilled screw holes from the underside and then applied the glue and used the screws to “clamp” it. Once dried, if you don’t want the screws, you can just remove them. Mine were as big as 6’ x 24” and haven’t had any problems. Ran 1×2 along the outside for edging (attached with biscuits and glue).

As for a caul, it’s been a while since I made mine, but i just used 2×4 and taped a 1/4” spacer to it towards one end and ran it through the table saw with the “spacer” edge against the fence (and the fence set at 3.5”). You end up with a board 1/4” shorter or so at the end, tapering to 0 at the center. Make sure you tape the spacer so that it doesn’t end up off the fence before you get to the middle of the board. Flip it over and put the spacer on the other end and do it again. Do that for two pieces and you have a caul. Took no time at all and no special effort. Ran off two sets of them in less than 10 mins. Have used them many times, work great.

-- "A hammer in search of a nail..." (

View FirehouseWoodworking's profile


722 posts in 3474 days

#10 posted 09-02-2010 02:29 AM


Not to distract from the discussion, but have you thought about buying a slag solid core door and cutting it down to size? Might be simpler.

Cost might be a wash or even cheaper than MDF if you ask for a damaged door at a big box store. They like to get rid of the damaged ones as quick as they can.

You could band it with hardwood and cover the top surface with laminate.

Just sayin’.

-- Dave; Lansing, Kansas

View Rick  Dennington's profile

Rick Dennington

6301 posts in 3395 days

#11 posted 09-02-2010 02:44 AM

Greetings Mark, again,

Trust me….... the glueing and screwing method is the way to go… I’ve built 5 workbenches and 4 router tables doing it, and not one of them has come apart…..The last workbench I built for my shop I glued and screwed 4 sheets of 3/4” MDF together for the top…that’s 3” thick…..This bench top is 42” wide and 101 ” long, and bud, it is stout and solid…... but more importantly it is fast and quick to do, and will be dead-flat.. My bench has been together now for about 2 years, and the top is dead-flat after all that time…. go for it…

-- " At my age, happy hour is a crap and a nap".....!!

View ChuckM's profile


615 posts in 3867 days

#12 posted 09-02-2010 02:51 AM

In my opinion, you don’t need a dead flat surface for the whole top. The critical area is the area where you put the fence and the router. If that section and the surrounding area are flat, you’ll be fine in most cases. The MDF is flat but not necessarily so over its entire surface and the same goes for the work pieces (large) that you’ll work with. Even if your router top is totally flat, other factors such as the fence setting, the operator’s skill, stock preparation (true & square) etc.may affect more the outcome of your cuts. So don’t lose any sleep if the router tabletop isn’t 100% flat. Trust your eye and hands … if it looks flat, if it feels flat, that’s flat enough.

-- The time I enjoy wasting is not time wasted

View Mark Colan's profile

Mark Colan

211 posts in 3046 days

#13 posted 09-02-2010 03:24 AM

@phlepper: thanks for you shortcut on making cauls. I’ll try it.

@FHWoodworking: it’s a good idea, but I have already bought the MDF. I do intend to laminate and band it when two slabs are solidly glued together.

@RickDennington: presumably you drilled pilots and countersinks for the screws to ensure you did not crack the MDF? Or maybe they are far enough from the edge that there’s no chance?

@ChuckM: I was thinking the same thing. I can get a big enough critical area by using the steel table of my table saw.

-- Mark, hack amateur woodworker, Medford (greater Boston) MA

View Mark Colan's profile

Mark Colan

211 posts in 3046 days

#14 posted 09-02-2010 03:28 AM

This lumberjocks board ROCKS.

I joined recently and put just a bit of info on my profile. 13 people (so far) welcomed me to the board!

Then I posted this question 12 hours ago. It has had 229 views, so far. 11 replies (excluding my own), including one from a guy who INVITED ME OVER to help me make cauls.

I mean, woodworkers are a friendly lot and all, but this is blowing my mind!

-- Mark, hack amateur woodworker, Medford (greater Boston) MA

View stratiA's profile


101 posts in 3576 days

#15 posted 09-02-2010 04:41 AM

Ironic that I would find this post. Today I glued together 3 2×4 pieces of mdf. I used yellow glue and cauls and shot a couple of brads in the corners to keep them from shifting. no problems at all.

-- Strati Alepidis, Burlington, Ma, Member Red Sox nation

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