Several questions and comments...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?
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.