My MDF Questions: #1 How Do You Seal It?

[Tom Overthere]

I've been accumulating LOTS of questions about how to use MDF, and if you guys don't mind I'd like to launch a series of them - each question as a separate thread. I want to start using a lot more MDF, for shop-made jigs/fixtures and for work piece substructures.


Q1: What's the most effective and efficient way to seal MDF against moisture when it's used as either a shop jig or as a substructure for veneered furniture pieces? And yes, I need to apply hardwood veneer to one broad face, and probably apply edgeband to at least one edge.


I always considered MDF to be ultra stabile, but recently I heard someone say it must be carefully sealed to avoid every possible dampness - even ambient humidity :eek:!!!. Does that sound right? Does MDF really react so much to changes in humidity?

Also, I read about a sizing for MDF edges consisting of a 50/50 mix of wood glue and water, double-coat painted on the edges. Any opinions for or against this method would be much appreciated.

Thanks

[David Christopher]

for shop fixtures I use sanding sealer, sand it wipe it down and use it....I have had some of these fixtures for years and they are still like new

[glenn bradley]

Like David I seal mine. I use a thin dewaxed shellac. I've had them out in the gara . . . er, shop for years with no problems. For sliding jigs I do a few coats, sanding in between to create a smoother and smoother surface. I sand with 400 or 0000 wool on the last coat and then paste wax. The underside of my largest sled is done this way and has been used for years with just routine re-waxing.

[jeff begin]

I've used both shop-made glue size (1:10 glue/water ratio) and a 2 lb cut of shellac with good success. Though I do it mainly to reduce friction on jigs, drawer bottoms, etc.

However, I think someone might have overstated the problem to you. By weight, there's more adhesive than cellulose fibers in MDF so it's very dimensionally stable. According to "Understanding Wood", MDF only has an average linear expansion of 0.2-0.4% when going from 50% to 90% RH. Most domestic hardwoods are in the 7-10% range.

Direct contact with water is a different story though. Expect MDF to swell by 2-10% when soaked in water for 24 hours.

[Larry Edgerton]

I use West System epoxy. It is great for the running surfaces of jigs because you get no wear. It hard surfaces the MDF so that bearings do not eventually sink in. It soaks right in to about a 1/4 depth on end cuts and it will then sand up like glass, and paint will not disappear.

I have sign letters that have been outside in our harsh climate for 20 years, and not only are they in good condition, the paint has not peeled in that time because of moisture.

[Frank Drew]

Tom

I agree with the others that you should seal particularly the edges of MDF.


And yes, I need to apply hardwood veneer to one broad face

Best practice would be to apply veneer of similar thickness to both faces.

[Jerome Hanby]

For my quick and dirty jigs, I don't seal it at all. For things I plan to use long term and for projects, I use Bullseye shellac right out of the can, I think it's a two pound cut.

[Ted Shrader]

Tom -

For MDF jigs, break out the shellac. Wax the sliding surfaces (if any). For bent form laminations, wax the areas where there may be glue squeeze out and then cover them with Saran wrap or wax paper during glue up.

Ted

[Bill White]

Ah HAH! And now the quest has become more interesting. There is an outdoor quality mdf product available. I used it on the bungalow style columns for our home. It is called "Extera". (Usual disclaimer: no affiliation, blah blah.) Had some off cuts left, and used 'em in the shop for odds and ends. It is heavier and darker than std mdf. Just so you'll know.
Bill;)

[Chris Padilla]

I used whatever clearish like coating I have laying around (typically polyurethane) to seal MDF for long-term use.

DEFINATELY veneer both sides of an MDF substrate with the same thickness hardwood veneer. I've personally witness what a 1/16" piece of walnut veneer does to an MDF panel when it is applied to only one side: it bows it slightly! Consider further that this MDF panel was 50" x 17" x 1 1/4" thick (two sheets of 5/8" MDF laminated together) and I was mildy surprised it pulled it out of flat.

[Tom Overthere]

Thanks, guys. This is what I understand so far:


David: Sanding Sealer for shop fixtures. Sand it. wipe it off. Done.

Glenn: Dewaxed Shellac. Several coats, sanded.

Jeff: 1:10 glue-to-water with 2lb cut shellac (all mixed together?)

Larry: West System epoxy for hard surfacing a jig's wear surface, and for exterior apps.

Frank: Apply same veneer to BOTH faces

Jerome: Bullseye Shellac straight from can (2lb cut) for long-term-use jigs

Ted: Shellac and then wax the wear surfaces. Wax + waxed paper protects from glue squeeze out.

Bill: "Extera" external MDF, heavier and darker than standard

Chris: Veneer BOTH faces of MDF substrate.


Next issue: Quoting from my original question, "What's the most effective and efficient way to seal MDF against moisture when it's used...as a substructure for veneered furniture pieces?"

Which (if any) of the above suggestions provides a proper surface for affixing hardwood veneers (including some basic inlay work)?

I'm interested in MDF as a cost-cutting measure, so I really don't want to glue an expensive veneer to both sides of anything...

Assume I'm making a long, narrow table for behind a sofa, and that I want to apply veneer to the table's top and side panels. Do you guys really think I have to apply the same veneer to the underside of the table top and to the back surface of each side panel?

If I use plywood instead of MDF, would this not be an issue?

[jeff begin]

Jeff: 1:10 glue-to-water with 2lb cut shellac (all mixed together?)

No, either glue size or shellac. Glue size is a lot cheaper though, so I normally use that if I use anything at all. Though it tends to swell the MDF more than the alcohol from the shellac. I've never really had any issues with humidity affecting MDF though (even before I seriously got into woodworking, when I just tinkered).

[Chris Padilla]

Next issue: Quoting from my original question, "What's the most effective and efficient way to seal MDF against moisture when it's used...as a substructure for veneered furniture pieces?"

Which (if any) of the above suggestions provides a proper surface for affixing hardwood veneers (including some basic inlay work)?

I'm interested in MDF as a cost-cutting measure, so I really don't want to glue an expensive veneer to both sides of anything...

Assume I'm making a long, narrow table for behind a sofa, and that I want to apply veneer to the table's top and side panels. Do you guys really think I have to apply the same veneer to the underside of the table top and to the back surface of each side panel?

If I use plywood instead of MDF, would this not be an issue?

When I veneered my panels in my Tansu (http://www.sawmillcreek.org/showthread.php?t=92396) project, I simply rolled out (http://www.veneersupplies.com/product_info.php?cPath=86_37&products_id=382) my veneer glue (http://www.veneersupplies.com/product_info.php?cPath=86_40&products_id=1319), applied the veneer, pressed it. Done.

I also edge-banded the MDF substrate FIRST, then applied the veneer.

You don't have to apply the SAME veneer but it would be best to apply the same wood, just a cheaper cut of it, but it should be of similar thickness as well. What you want to create is BALANCE in your veneered piece. Otherwise, it could bend/bow on you if it is not secured. Perhaps your joinery or project will "capture" the veneered piece and keep it in check...perhaps it won't.

Plywood may have less a chance to bow/bend but again, you want to strive for balance. When you apply veneer to one side of the plywood, you are most certainly imbalancing the plywood.

Hopefully this provides you better insight.

[Tom Overthere]

Thanks, Chris. I'll have to research it more. If I have to veneer both sides, I think it'd make more sense to use cabinet plywood from the jump.