# Joining 3/4 MDF on edges



## inovermyheadagain (Aug 6, 2018)

HI,
Brand new to forums here, but everyone seems really helpful. I'm trying to create a single solid sheet of MDF 6.5 feet wide. (hanging artwork on it) I chose MDF because it's straight and dead smooth - great texture for a backdrop and should hold up over time. I couldn't get sheets wider than 4 feet, so cut it down to two sheets and now need to join together to make one large sheet. I thought I could butt them up to each other and screw on reinforcing boards on the back side to strengthen. 

When I tried that strategy with scrap (don't want to pay for my materials twice) I can't get a good clean joint - screws don't seem to pull the reinforcing boards and MDF all the way together tightly. (I was counting on the screws to create tight glue bond, and counting on the glue to do the long-term work.) The one time I tried pocket holes with MDF previously I was very unsuccessful, so haven't considered that option this time. Does anyone have any suggestions for creating a tight(ish) joint between the narrow edges of this material and get a reasonably precise edge? It doesn't have to be load-bearing or strong - just has to stand there and stay still.

I have never done biscuit joinery, but I do have access to a biscuit joiner - would that be a good approach for MDF? Is that a skill I can pick up quickly?

Finally, I don't have any clamps that would hold 6.5 feet together. Was thinking of using tow-straps (like tie downs for my truck) to create lateral pressure to hold the boards together while the glue dries. Has anyone done that? Or taken other non-traditional clamping approaches to long spans?


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## shoot summ (Feb 21, 2014)

inovermyheadagain said:


> HI,
> Brand new to forums here, but everyone seems really helpful. I'm trying to create a single solid sheet of MDF 6.5 feet wide. (hanging artwork on it) I chose MDF because it's straight and dead smooth - great texture for a backdrop and should hold up over time. I couldn't get sheets wider than 4 feet, so cut it down to two sheets and now need to join together to make one large sheet. I thought I could butt them up to each other and screw on reinforcing boards on the back side to strengthen.
> 
> When I tried that strategy with scrap (don't want to pay for my materials twice) I can't get a good clean joint - screws don't seem to pull the reinforcing boards and MDF all the way together tightly. (I was counting on the screws to create tight glue bond, and counting on the glue to do the long-term work.) The one time I tried pocket holes with MDF previously I was very unsuccessful, so haven't considered that option this time. Does anyone have any suggestions for creating a tight(ish) joint between the narrow edges of this material and get a reasonably precise edge? It doesn't have to be load-bearing or strong - just has to stand there and stay still.
> ...


Several questions and comments...

Assume you are going to paint this?

If yes then filler is your friend even if you do get a good joint.

From a clamping perspective when I put my 14' bench top together I screwed blocks on about 3-4" from the joint and used them to clamp the 2 pieces together, this method requires clamps less than a foot long. In my case I was using 2 sheets(double thickness) with an offset joint so I let the bottom piece provide the alignment. In your case you would need your support pieces to keep the surfaces flat to each other. You will have screw holes to fill...

When screwing into many hard surfaces the screw starting tends to lift your piece off of the surface, and it wont draw tight. I either back the screw out after the first tightening and do it again to cinch it tight, or I pre-drill the holes. I'm typically in a hurry so I used the first method most often.


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## inovermyheadagain (Aug 6, 2018)

Thanks! I like the clamping strategy - that sounds like it should be a lot easier than messing with strapping all the way around the work.


I hadn't planned to paint the board itself, but going to hang a wall mural onto it. It's going into my office, but the walls are concrete block and nowhere near smooth, so can't hang the mural directly on it. The painting is on a heavy leather-textured vinyl, so it should be forgiving of a bit of irregularity in the surface, but don't want a visible vertical line running up the whole thing if I can help it.



I did pre-drill the holes, and used coarse-threaded #10 screws, but just couldn't get the material to tighten down like I wanted. have been looking more into biscuit joints - I think they may do the trick if combined with your block-and-clamp technique. Can add blocks to back side of the material where holes shouldn't be a concern. Does MDF respond acceptably to biscuit joints?


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## Toolman50 (Mar 22, 2015)

MDF is available in 4’ X 8’ sheets. Compared to other types of material MDF is relatively inexpensive. It would best if you started with a full sheet so you would have no seem to worry with. 
If you insist on using what you have, I recommend a wide backer-board of 6” or so. Other thin strips on each edge will keep the frame balanced against the wall.


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## inovermyheadagain (Aug 6, 2018)

If I could get a 78 inch square out of a 4x8 sheet I would do it. But the geometry doesn't work out. I have to join 2 pieces to get that size unless I can find something at least 78 inches on the shortest side. (a 4x8 sheet is 30 inches too narrow.)


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## shoot summ (Feb 21, 2014)

inovermyheadagain said:


> Thanks! I like the clamping strategy - that sounds like it should be a lot easier than messing with strapping all the way around the work.
> 
> 
> I hadn't planned to paint the board itself, but going to hang a wall mural onto it. It's going into my office, but the walls are concrete block and nowhere near smooth, so can't hang the mural directly on it. The painting is on a heavy leather-textured vinyl, so it should be forgiving of a bit of irregularity in the surface, but don't want a visible vertical line running up the whole thing if I can help it.
> ...


Biscuits will work in MDF, but they will not force the two surfaces to be perfectly flat with each other, there is still a small amount of give.

Do you have a flat surface to work on? That might be part of the issue.

Depending on how much of a line you have, you might play around with running it horizontally instead.


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## Quickstep (Apr 10, 2012)

Do the fasteners need to be invisible? 

I think the problem that's causing things to not be flush is that when screws are driven into MDF, it makes a bulge that prevents the boards from pulling together completely. 

If the fasteners don't need to be invisible, I would drill holes through the board and the backer board, then sand off the bulges made by drilling. Then I'd use glue and machine screws with washers and nuts to hold it together while the glue sets. Once the glue dries, you could remove the machine screws and putty the holes if you wanted.


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## ducbsa (Jul 6, 2014)

Use this type if you want long clamps:

https://www.harborfreight.com/2-piece-3-4-quarter-inch-heavy-duty-cast-iron-pipe-clamp-31255.html


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## Packard (Jul 27, 2018)

You will have to add some structure behind the board or whatever you use to fill the crack will split when it is moved.

You can glue two layers of MDF with the joints staggered for the strength (it will be very heavy but dimensionally stable. Or you can glue the it to a plywood backer. Glue and screw from behind. 

Any edge to edge joint will develop cracks in the finish when you go to move the board to the final location. 

Use acrylic Spackle, which I think is less likely to crack than grain filler or joint compound to fill the joint. Any place where you've sanded through the glossy surface will need to be sealed if you are to apply a water borne finish. Otherwise it will "raise the grain" and make a very rough surface that will be very unsatisfactory. You can use Sealcoat shellac or BIN shellac based primer for that purpose.

If you have enough weights to weigh down the surface during the glue-up then you only need a few brads to keep things from sliding around. Sand bags are useful for this purpose. But you'd need a lot of them.

Short screws driven in from behind will do the same. You will have to drive them on 8 or 12 inch centers. The screws only need to penetrate the MDF about 3/8" and can be removed after the glue has dried.


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## FrankC (Aug 24, 2012)

It may be more pleasing to see a seam down each edge so I would use 3/8" material and add strips to the 48" panel on each side on the top layer, have the seam in the center of the back layer. If it is not going to be painted perhaps round the edges where they meet to appear as a flute in the panel.


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## Packard (Jul 27, 2018)

How wide do you actually need the sheet? Baltic Birch (a very high quality product) comes in 5 foot x 5 foot sizes.

I see it is now available in 5' x 10' sizes:

https://www.wolstenholme.com/plywood-products/specialty-panels/oversized/


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## Shop_Rat (Dec 22, 2015)

I'd probably cut down and attach a couple sheets of 3/8" drywall to a 1"X 3" backer frame and then finish the seam with tape and mud for a perfectly smooth solid surface.

I'd save the MDF for an MDF kind of project.


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## Packard (Jul 27, 2018)

Or build a frame and stretch canvas. I've seen 65" wide rolls by 18 foot lengths. 

A substantial frame would still be way lighter than MDF, and would have no worrisome seam. 

https://www.usaoncanvas.com/include/guide_large_scale_products.php

They show 5' x 12' sizes.


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## Shop_Rat (Dec 22, 2015)

He's asking about a 78" square solution. Four-by and five-by just aren't going to do it.

Your choices are really fairly limited for a perfect seam line. I can think of no way to get a "no-seam" line in two slabs of MDF without using some sort of system like is used in drywall finishing. Even bondo would crack at a seam line. You really need to bridge the seam with something like tape and then feather some goop into the boards. Or, again, use real drywall.


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## woodnthings (Jan 24, 2009)

*Aw heck, use plywood ...*

A completely different approach to get a 78" square piece, would be to use 1/4" plywood cut to 78", either 4 or 6 pieces and alternate the seams at 90 degrees. The 6 pieces would give you a 3/4" thickness and should be stable. Plywood won't care if you cross grain glue it, it's already that made way. :vs_cool:


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## andr0id (Jan 11, 2018)

Take a look at this video.








I think if you lay your panels face down on a flat surface and provide some weight at each screw hole, you can get your edge pretty flush since the clamps are not going to reach into the middle of a sheet. I'd put a pocket screw every 8 - 12". (I like overkill.)



While the pocket screws will hold your sheets together tightly, you might still need additional support behind your panel.


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## Kristensawdust (Aug 9, 2018)

Thinking outside the box here. What is the completed size of “surface area” that you need? My sister in law once bought a plain door and painted on it. She hung it on a cathedral ceiling living room. She needed artwork to fill the space. It had plenty of flat white surface area to paint on. Something like this


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## woodnthings (Jan 24, 2009)

*close, maybe*



Kristensawdust said:


> Thinking outside the box here. What is the completed size of “surface area” that you need? My sister in law once bought a plain door and painted on it. She hung it on a cathedral ceiling living room. She needed artwork to fill the space. It had plenty of flat white surface area to paint on. Something like this


He needs a 78" square. Two hollow core doors, 36" wide if glued together will be 72" wide and 80" long. They will be light and stable. There will be a single seam in the center, BUT if you glue in a 6" wide space there will be 2 seams, if the 78" width is required. The additional thickness will help keep it flat and the hollow core construction will still be fairly lightweight... :vs_cool:


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