Dealing with MDF shrinkage?

[flanajb]

Can anyone tell me whether MDF is prone to shrinkage and are all 18mm mdf boards equal?

Reason I ask is that I have purchased MDF from B&Q and my local builders merchant in the past and noticed that it shrinks slightly and the 2 part filler that I used to hide the screw holes becomes visible after it has been in the house a while.

This is really annoying, as I spend ages sanding and spraying to get a really good finish, only to then have the filled screw holes show.

This does also raise the question whether there is a better method for jointing MDF when making carcasses. Would a Domino jointer mean I could dispense with the screws?

Thanks

 

[Random Orbital Bob]

Can anyone tell me whether MDF is prone to shrinkage and are all 18mm mdf boards equal?

Very little shrinkage in my experience and MRMDF gives even less than the rubbish from the sheds. Nominal thickness is dependent on supplier. I've had it in thickness range from 18mm to nearly 20mm from various sources. The sheds are typically closer to 18mm which is a pain if you expect a 3/4" router bit to give you a snug housing!! Whats its much more likely to do is expand on contact with water rather than the other way around.

Reason I ask is that I have purchased MDF from B&Q and my local builders merchant in the past and noticed that it shrinks slightly and the 2 part filler that I used to hide the screw holes becomes visible after it has been in the house a while.

This is really annoying, as I spend ages sanding and spraying to get a really good finish, only to then have the filled screw holes show.

This does also raise the question whether there is a better method for jointing MDF when making carcasses. Would a Domino jointer mean I could dispense with the screws? The answer you're looking for is "yes you should go and buy a domino"
Thanks

 

[Stu_2]

+1 for the Domino. I also use pocket holes in conjunction with the Domino, too. Makes life a lot easier.

 

[custard]

If it's purely for jointing MDF panels I'd be inclined to go the biscuit route rather than get a domino.

1. Biscuits are cheaper
2. Biscuit jointers are cheaper
3. Biscuits are faster and more forgiving
4. Dominos are often a very tight fit, which can cause the MDF to swell slightly around the domino. Not usually a major problem but can cause fitting problems in a drawer cavity.

If you're in south east Dorset and want to try both dominos and biscuits to see for yourself then PM me and you're welcome to have a go in my south west Hampshire workshop.

By the way, another vote for branded, moisture resistant MDF from a reputable supplier. MDF from the DIY sheds just isn't that great, it's often not stored or handled properly and the basic material is frequently ropey...I've seen it contain chewed up bits of metal when it's been made from recycled pallets!

 

[flanajb]

If you're in south east Dorset and want to try both dominos and biscuits to see for yourself then PM me and you're welcome to have a go in my south west Hampshire workshop.

- Thanks for the offer. I have a biscuit jointer, but have never used a domino on a project before

By the way, another vote for branded, moisture resistant MDF from a reputable supplier. MDF from the DIY sheds just isn't that great, it's often not stored or handled properly and the basic material is frequently ropey...I've seen it contain chewed up bits of metal when it's been made from recycled pallets!

- Thanks for that. I had a sneaky feeling that the quality of the boards from the sheds is not as good as that from say Timbmet. My local merchants store their MDF boards in an outdoor shed which although water tight does not look the driest place to keep MDF

 

[Stu_2]

If you have a Selco near you, they sell decent quality Norbord Pro MDF, and MR MDF. You'd need to open a cash account, but they're pretty flexible. I'm not VAT registered but it wasn't an issue, and there's no minimum monthly spend or any conditions.

 

[flanajb]

If you have a Selco near you, they sell decent quality Norbord Pro MDF, and MR MDF. You'd need to open a cash account, but they're pretty flexible. I'm not VAT registered but it wasn't an issue, and there's no minimum monthly spend or any conditions.

I will give the Norbord a try next time I require some MDF. Just found that my local Jewsons stock it.

 

[Steve Maskery]

I once made some radiator covers out of MDF they looked fab. Until, that is, the heating came on, when they ballooned out like Steve Maskery's belly. When the heating went off, they moved back again, unlike Steve Maskery's belly.

I never did work out whether it was the heat per se, or the inside face being drier than the outside when it got hot.

 

[mailee]

MRMDF is really the only thing to use as it is pretty stable. I use Medite from my local timber merchants and get a pretty good discount for buying in bulk. It is more expensive than buying the 'fluffy' stuff from the sheds but saves on the cost of more paint to cover it. :wink:

 

[Mickfb]

Prime the screw heads before applying 2 part filler. Plus I agree use m/r mdf. The stuff you buy at the sheds are rubbish.

 

[Silas Gull]

I use drywall screws for mdf work. These can be driven into counter-bored holes and the heads concealed with dowel segments. For really strong joints I use 9 mm fields in 18 mm mdf but loose fitting. Quick spritz of water into the hole with a spray bottle and then coat the dowel in Gorilla glue and tap home. Rock solid.

 

[n0legs]

Prime the screw heads before applying 2 part filler. Plus I agree use m/r mdf. The stuff you buy at the sheds are rubbish.

Good advice here ^^^^^.

 

[Random Orbital Bob]

I once made some radiator covers out of MDF they looked fab. Until, that is, the heating came on, when they ballooned out like Steve Maskery's belly. When the heating went off, they moved back again, unlike Steve Maskery's belly.

I never did work out whether it was the heat per se, or the inside face being drier than the outside when it got hot.

What a coincidence....my belly has also ballooned...I must be buying my MDF from the same source as you :shock:

 

[Benchwayze]

C

This does also raise the question whether there is a better method for jointing MDF when making carcasses. Would a Domino jointer mean I could dispense with the screws?

Thanks

If you have to ask the question, you know the answer.

You could use dowels. You could use a Kreg jig, (pretty strong and easy. A bit expensive on screws for a big job. With MDF, I'd use the Kreg, (There are other pocket-hole jigs available!!!) only if the carcasses are being supported by other cupboards.

A biscuit joiner is quite adequate for holding MDF and ply carcasses. IMHO)
You can also use a rebated joint with screws, again adequate especially when being supported by other carcasses.
Then there's the Domino of course. A Domino also makes face-frames a doddle, so if you're looking for an excuse to buy some Festool, then you've found one!