How to deal with this MDF join

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I made some panelling in our bathroom a few years ago out of water resistant MDF. Stupidly, i didn't glue the join and just screwed two sections to a fish plate piece of MDF behind them.

I now find that that the join expands and contracts depending on the time of year. In the summer, when the heating is not on, it closes up to virtually nothing. In the winter, when the heating is on, the crack opens up to a mm or so wide.

I want to fill and paint the but need to deal with the expansion/contraction issue at the same time.

Can anyone suggest how to deal with it? Wood filler will not bond the crack and will only crack again. I'm also worried that cauk will be squeezed out when the MDf expands in the summer.

All advice appreciated

Photo below
 
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Lower, good evening.

How about considering fitting a cover strip, fixed to one panel not both?

Or if you can remove the top shelf could you slide a plastic "H" section into the joint?

Ken.
 
Can’t easily remove the top shelf. I could glue on a cover strip but I’m hoping to create a seamless join.
 
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That's why i'm hoping that someone knows of a miracle product that will fill and bond the join in one go :)

Annoyingly, where i glued and mechanically fixed the joints elsewhere there is no movement. Its just this join that i only screwed and the bit of MDF that i used as a fish plate is only 6mm thick so clearly not strong enough to just be screwed.
 
ok my thoughts
you have 3 choices
a vee groove then add other grooves around the panel to give a evenness and uniformity
a vee groove and a flexible caulk or filler as you need a bit off surface width to hide movement
or as said added beading around the panel to give a evenness and uniformity
 
V groove and then flexible filler might be the way forward. The join is straight so it should be easy to chisel out a decent recess without gouging.

Thanks to all that have contributed.
 
Epoxy filler will probably suffice if you can make the gap slightly wider using a multi-tool.

Not cheap though.

https://www.repair-care.co.uk/product/dry-flex-1-2-in-1-2/

The only reason I recommend the above is that it is one of the only tubes that can be used in a standard caulking gun. Most tubes require special guns which bump the price
 

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