MDF sunken joints

[marvinwright]

Hi,

A year or so ago I built myself a speaker enclosure from MDF. I glued and screwed the joints, the joints were filled with U-POL P38 filler. After the glue was set I removed the screws, drilled out and filled with dowels. Everything was sanded down and then I sealed the MDF with watered down wood glue. The finish I was after was a high gloss black, I'm no expert just a DIYer but I was pretty happy with the finish.
Anyway to my question, about 5/6 months after painting the joints began to appear, these had sunk, and where the dowels had been placed, these began to looked raised.

Below are a couple of pictures which show my issue.

I'm now thinking of making a new box but I don't want to make the same mistakes.
What causes this and how can I avoid it happing next time?

Any advice would be welcome.

Thanks,

Marv.

 

[Doggit]

I suspect that the body filler hasn't adhered to MDF properly, and the vibration from the speakers has shaken things lose. Did you clean the dust from the MDF before applying the filler. You might need to use the watered down glue trick to seal the MDF first before applying the filler, or make enquiries as to a more suitable wood glue.

 

[JohnD]

you can assemble it using battens inside, screwed only from the inside and not penetrating the outer surfaces. Or you can use a dowelling jig to make invisible joints, again, not penetrating the outside surface. Both are very easy, you only need an electric drill. Your joints should be very tight and not need filler. I expect vibration has loosened it, and your filler has shrunk, probably as it dried.

 

[Doggit]

He used body filler John, so it wouldn't have shrunk. It works pretty much the same as 2 part wood filler, so should have been fine - I've used it myself on wood repairs at times.

 

[JohnD]

but with either of my methods there will be no external holes, so no filler required. I am not much good at woodwork but have had good results, with the boards cut true and square.

I think my jig is from Eclipse.

Not quite the same as this, but I think comparable
http://www.ebay.co.uk/itm/Double-Dowel-Jig-/111920577520?hash=item1a0efc7bf0:g:5V8AAOSwB4NWzGsY

 

[marvinwright]

Thanks for the replies. The box still does seem to be air tight, although the joints have sunk it doesn't seem to be loose in anyway.

Yes, the MDF was cleaned and dust free before applying the filler. The joints were pretty tight when I glued them, not all joints required filler yet all joints show a very thin line. Could there be any possibility that its the glue I used, it was ordinary wood glue from Wickes?
It was a while now so I can't quite remember but after glueing it all together and filling I sealed and painted very soon after. I've read that MDF is very good at holding moisture, I wonder if it should be left for a while to fully dry out?

Marv.

 

[JohnD]

the edge join is very hard to hide, you might try running a slight bevel on it before assembly and make a feature of it. The vibration will tend to crack any hard filler or glue edge, which will be very thin.

 

[chappers]

As John says I would always expect it to be hard to make the edge join totally invisible with a hand painted finish and I would have left your screws in and filled the holes with car body filler, sanded back to 400 grit

 

[gregers]

could you not biscuit joint it all together?

 

[chappers]

think you would still have the same issues with the joint lines, the only real way to get a jointless gloss finish is to get it sprayed

 

[opps]

Thanks for the replies. The box still does seem to be air tight, although the joints have sunk it doesn't seem to be loose in anyway.

Yes, the MDF was cleaned and dust free before applying the filler. The joints were pretty tight when I glued them, not all joints required filler yet all joints show a very thin line. Could there be any possibility that its the glue I used, it was ordinary wood glue from Wickes?
It was a while now so I can't quite remember but after glueing it all together and filling I sealed and painted very soon after. I've read that MDF is very good at holding moisture, I wonder if it should be left for a while to fully dry out?

Marv.

MDF is in a perpetual state of expanding and contracting as it follows the relative humidity of the room.

The two pack filler is basically being pushed out of the hole as the MDF expands and contracts, the same applies to the joints where different end grains meet.

In an ideal world all of the edges would be mitred but that isn't always an option. Could you not design "shadow lines" next time? you will never eliminate them so why not make a feature of them?

Swollen screw heads will always reappear- try using a biscuit joiner and micro pinner (23 gauge) and then fill the tiny holes with Red Devil OneTime.