MDF shrinking?

[savantstrike]

So I'm going through the posts and I just read one that MDF will shrink over time with **** joints? Is this true? If so, how long do I have to wait before it's properly shrunken down and will not be a problem??

I was going to put some 1/4 veneered plywood on a box I just finished, but now I wonder if I need to wait for that.

[thylantyr]

>>So I'm going through the posts and I just read
>>one that MDF will shrink over time with ****
>>joints?

One post and there is fear?

more beer... there is no fear.

[blair]

I've built a ton of stuff out of MDF. Not speakers usually, but I have never had a problem with my seams getting misaligned over time. Even working on them out in the cold and bringing them into the warm house. Make sure there is enough glue, and if you are really afraid of this, put small corner braces along each edge made out of rectangles of MDF.

Blair

[AJ]

It can shrink over time if it has some moisture in it. If you live in a dry climate you don't have to worry. If you live in a humid climate you may want to look into something like Aaron hero uses to dry out his cabinets. I think it was like some kind of forced air space heater...

[savantstrike]

Sounds like I should be okay with the contact cement and 1/4" ply keeping it together... I'm not in a particularly humid climate.

> It can shrink over time if it has some
> moisture in it. If you live in a dry climate
> you don't have to worry. If you live in a
> humid climate you may want to look into
> something like Aaron hero uses to dry out
> his cabinets. I think it was like some kind
> of forced air space heater...

[bobbarkto]

It can be a real problem with high gloss finishes.

The joint lines _will_ telegraph as the mdf gains and loses moisture.

This is almost unavoidable, unless you work and keep the mdf in a tightly controlled environment (not more than ~5% seasonal variation in RH and temp).

[savantstrike]

Hmm... does this specifically affect veneer (either iron on or contact cement on), or my 1/4" plywood option?

> It can be a real problem with high gloss
> finishes.

> The joint lines _will_ telegraph as the mdf
> gains and loses moisture.

> This is almost unavoidable, unless you work
> and keep the mdf in a tightly controlled
> environment (not more than ~5% seasonal
> variation in RH and temp).

[bobbarkto]

That depends.

Assuming that RH changes seasonaly:

If it is just thin veneer with a high gloss finish, yes, absolutely will show. Glue and finish will not matter.

Paper or other backed veneer might mask it, depending on the severity of the changes in RH and finish used.

Several layers of veneer or very thick veneer (>1/8") will usualy hide it.

1/4" plywood skin over an mdf box should completely mask the effect. You won't see the mdf movement.

> Hmm... does this specifically affect veneer
> (either iron on or contact cement on), or my
> 1/4" plywood option?

[boso]

> That depends.

> Assuming that RH changes seasonaly:

> If it is just thin veneer with a high gloss
> finish, yes, absolutely will show. Glue and
> finish will not matter.

> Paper or other backed veneer might mask it,
> depending on the severity of the changes in
> RH and finish used.

> Several layers of veneer or very thick
> veneer (>1/8") will usualy hide it.

> 1/4" plywood skin over an mdf box
> should completely mask the effect. You won't
> see the mdf movement.

I have used an extra layer of veneer with the grain going the 90 degrees to the final veneer on speaker cabinet tops. This takes care of the problem for me. I think the old timers called it cross banding.

For piano finishes I think a 45 degree miter joint is the safe bet but I don’t do piano finishes.

[bobbarkto]

Cross banding helps for sure. May even eliminate the problem. Even on a high gloss finish.

Miters are prefered. But if the design says no to miters...

> I have used an extra layer of veneer with
> the grain going the 90 degrees to the final
> veneer on speaker cabinet tops. This takes
> care of the problem for me. I think the old
> timers called it cross banding.

> For piano finishes I think a 45 degree miter
> joint is the safe bet but I don’t do piano
> finishes.

[wingnut]

Bob is correct in what he says. However, a good finish will slow this process to point of it not being an issue. MDF is no more susceptible to this problem than solid woods, or other manufactured wood products. The only thing you might want to change, environment-wise, if you live in an area that experiences extreme cold in the winter is add a humidifier to your heating/cooling system. The worst effects of this phenomenon are experienced in cold climates where the relative humidity can equal the dryest deserts.

[bobbarkto]

Actualy, mdf moves much more in thickness than any other common building material (it also reacts more quickly to changes in RH than most other common materials).
And therin lies the problem when using a **** joint.

Length and width movement in mdf is identical and somewhere in the middle area compared to other common building products.

*No finish will prevent seasonal movement.

Daily, weekly, even monthly variations can be controlled enough with proper application of an appropriate finish. But if you live where climate changes are dramatic and lasting, then no finish will eliminate movement enough to keep **** joints from showing in a high gloss finish.

> Bob is correct in what he says. However, a
> good finish will slow this process to point
> of it not being an issue. MDF is no more
> susceptible to this problem than solid
> woods, or other manufactured wood products.

[wingnut]

I only speak from my experience, but I've never had a problem with veneered mdf IF it had a finish on all sides. I've encountered slight problems with pieces that had exposed wood, even if only a small area on the bottom, for example.

On the other hand, my house is well humidified in the winter time and I'm sure this helps the situation.

T. Todd

[bobbarkto]

I know. Many people don't experience a problem. But for those that do...

A less than mirror like finish goes a long long way in keeping the problem invisible.

For example, I have two sets of 2-ways, same relative size mdf boxes.

One is finished in cherry veneer with a gloss tung oil finish, applied only to the outside. Gloss level is med. high, not deep. It looks great all year. Yet I can measure the seasonal movement.

The other boxes have a sycamore veneer, finished in a highly polished gloss clear acrylic, min 3 coats finish applied inside and out. These show the seasons, clearly. In the dead of winter the top panel shrinks and the sides stick up. In the most humid part of summer the top panel raises a little. For six months of the year they look great.

Black is the worst!

[boso]

> Actualy, mdf moves much more in thickness
> than any other common building material (it
> also reacts more quickly to changes in RH
> than most other common materials).
> And therin lies the problem when using a
> **** joint.

> Length and width movement in mdf is
> identical and somewhere in the middle area
> compared to other common building products.

> *No finish will prevent seasonal movement.

> Daily, weekly, even monthly variations can
> be controlled enough with proper application
> of an appropriate finish. But if you live
> where climate changes are dramatic and
> lasting, then no finish will eliminate
> movement enough to keep **** joints from
> showing in a high gloss finish.

I think just putting water base glue on the edge of MDF will make it swell just a little bit.

[bobbarkto]

For a time, yes, absolutely.

You'll see this as a sunken joint line when you trim and sand too soon after glue-up.

> I think just putting water base glue on the
> edge of MDF will make it swell just a little
> bit.

[Andy_G]

and the last thing we want to see is a crack in a **** joint.

[wingnut]

> and the last thing we want to see is a crack
> in a **** joint.

I prefer joint cracks over **** cracks, though -- unless you're talking my wife;D

[jtaylor]

> It can be a real problem with high gloss
> finishes.

> The joint lines _will_ telegraph as the mdf
> gains and loses moisture.

> This is almost unavoidable, unless you work
> and keep the mdf in a tightly controlled
> environment (not more than ~5% seasonal
> variation in RH and temp).

So now I need a Enviromental clean room to control the temp and humidity in my HT. How much will this cost me???? Just kidding. Store your wood materials together in the same room so the materials become accustomed to the temp and humidity should work pretty well.

The number one sin is to put hardwood grains perpendicular to another in a piece of furniture. We had a table built incorrectly and the seam split and one of the pieces would shrink dramatically each winter by as much as one inch.

Keep MDF flat while working with it and you shouldn't have any problems. Working with Bacon boards is no fun.

J. Taylor's 2 cents

[boso]

> So I'm going through the posts and I just
> read one that MDF will shrink over time with
> **** joints? Is this true? If so, how long
> do I have to wait before it's properly
> shrunken down and will not be a problem??

> I was going to put some 1/4 veneered plywood
> on a box I just finished, but now I wonder
> if I need to wait for that.

Don’t let MDF rest on its edge against a concrete garage or basement floor especially if the concrete is not sealed. Lots of moisture moves through concrete and wicks into the MDF. Put down a few small strips of wood or plastic to rest the edge of the MDF on.