[shoot summ]

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?

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.

 

[shoot summ]

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?

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.