How do seal MDF?

[Mark Largent]

I've built my own version of Norm's router cabinet, and all that's left to do is the top. I've glued together two pieces of 3/4 MDF and I plan to band it with a 3/4" piece of hardwood trim, cover it with laminate, and cut the hole for my Incra router plate.

From what I've read, I need to be concerned about sealing with MDF so that it does not absorb moisture. I've got three questions I'd really appreciate help with:

1) Do I in fact need to seal the MDF?

2) Do I need to seal all six sides, or just the bottom?

3) What should I use to seal it?

Thanks for your help,
Mark

[Ellery Becnel]

Mark,
I asked a similar question about sealing MDF for my project. A drum sander, Stockroom supply V-drum style using MDF disks.
I never got a reply. I hope you do. If I knew I certainly would share my findings.
Ellery

[glenn bradley]

IF you live in a climate that requires full sealing, I would spray both surfaces prior to assembly . . . I know, thanks a lot, huh? I seal MDF with de-waxed shellac. Some pieces like false fronts, get sealed all around. Parts like the carcass get sealed outside and on edges but, the insides only get a lick and a promise. These have been in service for years without issue but, I live in a desert basin with a narrow humidity swing.

253290 . 253291

This roll-around unit has been in use since 2005:

253293

[Bill White]

Second the shellac. After that, a good coat of wax gives ya a smooth finish.
Slipery-slide is a good thing.
Bill

[pat warner]

If you're looking for dimensional stability with some finish, that quest is futile.
It will take longer for your sandwich to reach a new EMC but it will arrive nevertheless, as the weather changes.
Chemical protection and abrasion resistance yes, but no protection from dimensional changes from weather.

[Cody Colston]

Since you are putting laminate on the top, seal the bottom well with shellac or more laminate to balance it out. Otherwise, the top may not stay perfectly flat. I wouldn't worry about the edges since they will be covered with the banding.

[Mark Largent]

Sounds like shallac on the bottom (and might as well do the sides), then laminate on the top. My worry with laminate on the top and nothing on the bottom is that it would eventually bow.

But, I'll shallac it and let you if and when I have a problem with it. Thanks!

[johnny means]

sealing the bottom won't prevent bowing the laminate on top will not expand while the MDF will still expand and contract. Laminate both sides.

[Thomas Canfield]

I find that using 2 liberal coats of thin shellac does a good job of sealing MDF. I usually light sand with 220 grit after each coat and then you can wax. The surface is slick and the thin (1:1 cut with DNA) will penetrate the MDF pretty deep.

[Peter Kuhlman]

My experience is shellac on both sides works great. Really toughens up the MDF. Now with laminate, you really need it on both sides to make it stable. Never see a manufactured router table top with laminate on just one side for a reason!

[Phil Thien]

sealing the bottom won't prevent bowing the laminate on top will not expand while the MDF will still expand and contract. Laminate both sides.

This. You MUST apply laminate to BOTH sides if you want any chance at all at keeping a flat surface.

[Jim Neeley]

I applied laminate to both sides and maple trim around the edges on my 28"x52" RT when I made the table (4 years ago) and it's totally flat. The core was MDF glued up to 1-1/2" thick.

Jim in Alaska

[Rich Engelhardt]

I always seal all six sides of MDF.

You know how you always seem to have a little of this and a little of that finish left over when you finish a project?
I dump all those leftovers into a "slop" can.
I use that to put a sealer on things like MDF.

I run a "slop can" for waterborn cleat, solvent based clear and waterborn paints.

[Mike Heidrick]

I had a one laminated sided MDF extension table on my delta saw built norm style for 3 years with zero movement. YMMV.

[Mark Largent]

I applied laminate to both sides and maple trim around the edges on my 28"x52" RT when I made the table (4 years ago) and it's totally flat. The core was MDF glued up to 1-1/2" thick.

Jim in Alaska

This was what I was hoping to hear - thanks Jim!