MDF sunken joints

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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.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.


I'm now thinking of making a new box but I don't want to make the same mistakes.
Instead of using screws then replacing with dowels I can use internal batters to construct the box.
Regarding the joins appearing I've read a few posts here and using a wood hardener seems one possibility that may help, the posts I read are quite old so I was wondering if there is a better approach that I can take to try and minimise this?

Any advice would be welcome.

Thanks,

Marv.
 
I used butt joints but decided to go with the screws while the glue set, I think next time I'll definitely avoid screws.

What kind of paint finish have you applied?

After filling and sealing, I primed with 2 coats of paint, flatted the paint, sprayed a lot of black coats and flatted the paint, added a clear coat spray lacquer before finally polishing it with fine compound polish. As I mentioned the finish was great at the start with no signs of joints at all. Admittedly the finish was not particularly hard wearing so possibly the lacquer was not great, not sure if there was something else I could of applied for a harder finish.

Thanks,

Marv.
 
I used butt joints but decided to go with the screws while the glue set, I think next time I'll definitely avoid screws.

What kind of paint finish have you applied?

After filling and sealing, I primed with 2 coats of paint, flatted the paint, sprayed a lot of black coats and flatted the paint, added a clear coat spray lacquer before finally polishing it with fine compound polish. As I mentioned the finish was great at the start with no signs of joints at all. Admittedly the finish was not particularly hard wearing so possibly the lacquer was not great, not sure if there was something else I could of applied for a harder finish.

Thanks,

Marv.

Maybe not so much harder as more flexible? Anything I've built with MDF, seams reappear at some future point. My TV false wall was perfect.... For up to 12 months. After that, the joints reappeared.

Another option would be to cover it in paper or cloth - like with an old-skool model aeroplane. You can trip out on the celulose plus aromatics whilst doing it too. :)
 
The less different materials in the joints the better. Don't use wood hardener, that was found to cause cracking of the material after time.

The usual cause is PVA glue allows creep as it is slightly flexible/rubbery. On top of this the joint is drying out from the water in the glue and its just a matter of time before the lines show.

Butt joints are fine, but clamp them. Cascamite is much better than PVA as it's rigid, but if you want to avoid any moisture takeup from the glue then epoxy is the way to go. But again it must be a rigid type.

I usually use Cascamite and leave the box for some weeks in the house before flush trimming with a router, so the joints have stabilised by then. For a quicker turnaround then use epoxy.

Sealing the wood with PVA probably didn't help either. Following the same mantra avoid PVA or proprietary (water based) sealer for sealing the MDF, automotive 2k high build primer applied with roller (spraying is dangerous) works very well.
 
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MDF is actually Really Crap material Fine sawdust in a Formaldahyde glue soup.
Humidity (from ambient air even) messes the stuff (it swells).
More than likely you are seeing the MDF expand .. rather than the bits shrinking
Stupid cheap Particle Board is a better speaker build material.. Yes!
In the very least it doesn't suffer humidity instability and if Veneered with some ply, outperforms even Birch plywood.
 
While there's rather a lot more work involved in prepping for a high quality paint grade finish on faces and particularly profiled edges of PB, I'd have to along with Bare on it making for a better sounding speaker enclosure than MDF.

Then of course, for sonics there's Baltic birch plywood in numeruous various identities/brand trade names - not without its own issues if looking to finish in other than clear topcoat, or veneer, etc.
 
A butt joint will show through paint no matter what. Best to know this and work with it. A round over that ends right on the joint will do a good job of hiding any telegraphing. I often chamfer the edges before assembly and am left with a purposeful v groove running the length of the joint....at least it looks like it's meant to be there.
 
The one comment that I haven't seen is that you need to seal the inside of the enclosure just as much as you do to the exterior surfaces. If the inside is left unsealed and that means every surface including the cutouts for the speakers the mdf will absorb moisture from the air. I have done piano black enclosures with mdf and if you fail to seal the inside with the same number of coats of sealer you will always have problems. Use a high quality epoxy or urethane base coat and make sure you get every surface crack or joint. Do not sand through this base coating and you will not see the same problems you are seeing. This is strictly a swelling of the mdf with the changes of humidity that are unavoidable. Perfect sealing of all surfaces is the only answer besides what Evanc has just said about highlighting the joints if you don't seal the interior surfaces or the common solution of using veneer over the mdf to hide the surfaces.
 
If the box is so small you can't get inside the box to brush then just pour some of whatever sealer you are using inside the box and roll the box around to coat the inside and just pour out the excess. A bit messy but it will seal the inside and that is the ultimate goal, to seal everything with a contiguous coating. Who cares if there are any runs on the inside, just get all the surfaces sealed so no moisture can get into or out of the mdf after assembly. If you use screws and leave them in and seal over the top of those with a polyester type putty don't forget that this will shrink over time of at least a week or two. You usually have to refill these types of materials at least once after allowing for this shrinkage. Painting mdf is a tricky endeavor for sure, not for someone who wants to get it done fast. JBL/Harmon did many boxes with mdf but they actually used hot melt glue for assembly and didn't use butt joints, they used fold up boxes so the joint was at the corner. I still say the easiest solution is to use a veneer over the top of the mdf for best surfaces. Many companies use a melamine type of surface on the outside of the enclosure and then paint that surface after assembly but this does require some good wood working skills to get tight joint fitments, usually done with cnc routers.
 

OldMike2

Member
2009-03-14 1:21 am
Marvin,

You have two things going on based on your pictures, differential expansion between the dowels and the mdf and differential expansion between the thickness and face dimensions of the mdf.

I built 8 subwoofer boxes from 1 inch MDF and put on a piano black finish.

Step 1 is let your mdf acclimatize to the environment they will be in for at least 3 weeks. If you can use internal battens and screws so much the better.

There is a lot of good advice in the above responses, as said above you need to seal both sides of the mdf to minimize future humidity effects.

Time is your friend, I let my glued up boxes stand for a month before final trimming of the woodwork, routed round overs on the outside edges.

I used automotive filler to fill imperfections, and screw holes, multiple thin coats with at least 3 days between each one and three weeks after final fill application before sanding.

I used automotive primer, spray cans, and built up five to six coats, using 400 grit wet and dry paper to sand between every 2 coats. I let the boxes stand for a month after the last two primer coats before sanding. I also primed the inside of the boxes to seal them.

I used a similar timed approach when I applied the colour coat, flat black paint, six to eight coats.

The final finish was spray bomb lacquer, again building multiple thin coats over a one month period. My recall is that I applied ten to twelve thin coats.

I waited a further month after the last two finish lacquer coats before final sanding and polishing. My total elapsed calendar time was over six months.

I built the boxes some eight years ago and the boxes still look perfect. I do have the advantage of living in a low humidity climate, Alberta.

Last few points make sure the fillers and paints are compatible and if at any point you sand through to the bare mdf you literally start over.

Tedious and time consuming but it works.
 
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The less different materials in the joints the better. Don't use wood hardener, that was found to cause cracking of the material after time.

The usual cause is PVA glue allows creep as it is slightly flexible/rubbery. On top of this the joint is drying out from the water in the glue and its just a matter of time before the lines show.

Butt joints are fine, but clamp them. Cascamite is much better than PVA as it's rigid, but if you want to avoid any moisture takeup from the glue then epoxy is the way to go. But again it must be a rigid type.

I usually use Cascamite and leave the box for some weeks in the house before flush trimming with a router, so the joints have stabilised by then. For a quicker turnaround then use epoxy.

Sealing the wood with PVA probably didn't help either. Following the same mantra avoid PVA or proprietary (water based) sealer for sealing the MDF, automotive 2k high build primer applied with roller (spraying is dangerous) works very well.

Hi richie00boy,

I'm just in the process of building my enclosure, I'm going to try the Cascamite, how did you apply it, with a brush or roller?

Thanks,

Marvin.
 
Hi,

So I applied some cascamite to part of my enclosure as I need to seal, prime and paint before I finishing constructing the ports. I've included a couple of pictures below, one in particular is a close up of the joint which shows small hairline cracks in the glue, is this normal, should I be worried?

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.


There seems to be an art to mixing it, my first mix was all lumpy, but second nice and smooth, think I've got the idea though.

Any advice would be appreciated.

Thanks

Marvin
 
Hey, thanks for getting back to me. Its funny, I partially sealed the inside with cascamite when I sent the above post. I then re-read your earlier post and figured out that's not what you meant :headbash:

Here are the links again, hopefully you can see them now.

https://drive.google.com/open?id=0B4JRjS81iYcjMjRlY29qR1JoZGc
https://drive.google.com/open?id=0B4JRjS81iYcjUVp2bDRNR3VDb00

I've got one box constructed now, no filling, sanding or sealing done.
I want to be patient, I can afford to be as I have another box to build so I want to kind of follow a process.

I'm planning on doing something like this.

Let the box stand for a week to let the joints fully cure and dry out, if there's some hot weather (unlikely) I might stand it directly in the sun to help.

Fill any gaps, leave for a few days

Sand filler and route joints, going to round over all edges.

Seal the entire box, possibly going to use Zinsser B-I-N primer/sealer with a roller, will do two coats and then leave for a few days.

Sand the entire box ready for painting, I'm not sure what finish I'm going to apply yet, it will be black but I'm not sure what to go for, probably be just black gloss. I've previously built a white table using ronseal weatherproof exterior high gloss paint, it came out really well so was thinking of using the black high gloss.

Anyway, any advice is welcome.

Regards,

Marvin.
 
Marvin, Just remember to give everything time to shrink. The body filler will shrink and it should sit for up to a week before you try and paint over it. Even an epoxy primer will shrink given the time. I worked in custom cars when I was much younger and you always wanted to give everything plenty of time to shrink doing each step. Given a choice I would use a polyester two part primer coat over an epoxy for the simple reason that epoxy actually absorbs water. Whatever you use just take your time.
 
If you used cascamite for the joints and use 2 part car body filler rather than the tub ready for use wood filler, you should have a good result with those steps you outlined.

What gaps are you expecting to see? A box should be tight and not have any gaps.

There are a couple of hair line gaps, I really could have done with more than 4 clamps to be honest on a box this size.
Regarding filler, I was planning to use U-POL P38, will this be ok or would you recommend something else?

Kindhornman, thank you for the advice too!

Marvin.
 
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