A solution to MDF expansion on joints, translams etc.

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Wow shinobiwan--that is some beautiful work.

I used to face similar frustrations as those expressed here. I found a great solution:

Build a butt-jointed inner box that is glued and screwed. Make it nice and square. Glue and screw some nicely veneered outer panel onto the inner box. The screws go in from the inside. Just don't go all the way though your outer panel. The corners at the top and sides are mitered 45 degrees, so you don't have any endgrain showing.

The problem with this--you have to have a super square tablesaw to get perfect panels that fit perfectly.

The solution for those of us less talented than Shinobiwan:
Make the panels slightly oversized so that they hang over the front and back. Trim with a router.

Make the front baffle out of solid wood.

The back panel gets painted. Yes, you can see endgrain on the back panel. Nothing is ever perfect, I can live with that flaw.

I have some pictures of the results of this technique, I will post them if I can find them.

JJ
 
I see what you are suggesting, although veneer may not cover what I'm thinking of doing. Some of the options I'm looking at ....

This is for a pair of sub boxes.

I may have to look at fillet edges rather than chamfer.
 

Attachments

  • jointing.gif
    jointing.gif
    4.3 KB · Views: 604
Disabled Account
Joined 2007
ShinOBIWAN said:
I know a couple of folks (not in the UK climate) who've said they don't need to use anything more than good primer and thorough filling/sanding prep.

Might I be one of those :D


Ed LaFontaine said:
You always have the option to emphasize the joint. A saw kerf is a simple way of doing this. The options go on from there. An inlay of contrasting wood will work.

:up: These are good options too, used on a lot of commercial stuff.

The best bet to easily avoid the problem is tight fitting mitre joints, with no end grain exposed at all. Difficult to do for multifaceted boxes (like Ant's above), but not impossible.

Avoid, like the plague, water based finish on unsealed MDF. This is anything that have water in it, even waterbased "sealers". Biggest offender: thinned PVA.
 
I did that on the first set of MDF boxes I built. I just veneered over the roundover. In winter, when everything is really dry, I can visibly see the transition. Maybe if I had sealed it really good before veneering, I wouldn't get that line.

That is one of the problems that prompted me to start using the methond I mentioned earlier.

JJ
 
Disabled Account
Joined 2007
paulspencer said:
What about this.

Chamfered joint with a large 25mm (1") round over bit, then the join is given a groove that is filled with Builders Bog - 2 pack filler (shown as white solid hatch).

Has anyone tried this or similar, and does anyone think it will work or not work?


The problem with most glue joints is lack of clamp pressure. Apply ample glue, then us LOTS of clamps and LOTS of pressure. It is virtually impossible to apply too much pressure with ordinary clamps. You don't need to worry about squeezing out all of the glue.
I have only ever noticed this problem when I was a bit slack about clamping.
 
paulspencer said:
What about this.

Chamfered joint with a large 25mm (1") round over bit, then the join is given a groove that is filled with Builders Bog - 2 pack filler (shown as white solid hatch).

Has anyone tried this or similar, and does anyone think it will work or not work?
45 degree mate seems to provide about the same way of expanding, thus reducing mismatch caused by expansion.
 
Disabled Account
Joined 2007
richie00boy said:
I think adding another filler material with it's own expansion/contraction properties is just going to create problems.

Very well could create 2 lines instead of 1. :bawling:

soongsc said:

45 degree mate seems to provide about the same way of expanding, thus reducing mismatch caused by expansion.


There is truth here, but everything, absolutely everything shows in ultra high gloss.
Best approach (speaking from my own experience) is to use the right glue, make nice tight joints, clamp like crazy, let dry an adequate time (overnite) and sand, sand, sand and then sand some more.
 
MJL21193 said:


Very well could create 2 lines instead of 1. :bawling:




There is truth here, but everything, absolutely everything shows in ultra high gloss.
Best approach (speaking from my own experience) is to use the right glue, make nice tight joints, clamp like crazy, let dry an adequate time (overnite) and sand, sand, sand and then sand some more.
Well, the best work results from combining technology to acomplish a goal. No argument from me here. Some people just make the design so that there should be a line there, and you hide thin lines in a larger line. The line now becomes a design instead of a defect.

:D
 
In my case, I want to achieve the look of a box that is one solid block with rounded corners

How many corners do you want to round off? All 12?

When I have finished MDF by painting, sanding, repeat--I have found that it takes several coats to seal up the endgrain of MDF. But once it is done, it doesn't start to show through with changes in humidity.

JJ
 
Disabled Account
Joined 2007
paulspencer said:
I've seen it done that way, and it makes sense.

In my case, I want to achieve the look of a box that is one solid block with rounded corners as if it has been carved out of stone and polished. It gives a look of solidarity. I know it can be done, it's just a matter of figuring out how, and doing it without a factory!


I have done it several times, on several different things. Pic below is a HDF and MDF amp case. No lines anywhere.

I use yellow carpenters glue, BTW. This is PVA glue with aliphatic resin added to increase strength. Remember, don't be afraid to clamp it as tight as possible.
 

Attachments

  • im001621.jpg
    im001621.jpg
    97.2 KB · Views: 696
Lots of great advice thanks

my best results have come from these techniques.
tight joints
epoxy adhesives
lots of clamps
clean dust thoroghly especially end edges!
Resin gelcoat (like boats are made)
seal all surfaces( yes the inside of the box)
automotive laquers.

my worst results
poor joints
yellow wood glues
no clamps
enamel spray paints
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.