Seal MDF?

[Ken Fitzgerald]

I am just finishing my version of Norm Abrams "Deluxe Router Station". The fence is made of 3/4" MDF. The working face of the fence is covered with a laminate. The bottom, back and sides are bare sanded MDF.

Can I apply a fast drying polyurethane to seal it without causing it to warp or become crooked?

I live in an arid climate. Our average annual precipitation is only 13".

Or should I just let it remain unsealed?

[Bob Vavricka]

I would think any finish that is not water-borne would work fine on MDF. I made a cut-off station for my miter-saw with MDF and used shellac to seal it. I don't know about the poly, but it took about three coats of shellac before I was happy. MDF seemed to soak it up like a sponge. I would seal it with something to make the boards slide a little easier and easier to clean off. You might want to try some poly on a cut-off to see what you think.

[Phil Thien]

Before painting MDF, I often treat the edges with a dilute of water in white Elmer's Glue (The Glue-All). This treatment prevents the MDF edges from absorbing paint like there is no tomorrow. But another neat thing is, it really toughens up the edges of the MDF. Whereas untreated MDF can be pulled apart with your finger nails, the treated MDF is pretty tough.

So on jigs made w/ MDF, I now apply the 50-50 water/glue mix to the edges to make them stronger.

I apply it with a foam brush. After it dries for several hours, I go over the edge with finer sandpaper to smooth it out (the water/glue makes the surface kinda rough). One coat is all that is needed.

You might want to try it on some scraps and see how it works for you.

[John Coloccia]

I used to make acoustic guitar mold out of MDF and I would seal them with a good marine varnish. Makes for a pretty good utility finish on MDF, in my opinion. I would think that you'd want to seal it to keep it from warping, assuming the laminate is doing a pretty good job of sealing the face, but I think I know what you're getting at. You're concerned that in applying the sealer, that itself will cause it to warp, right?

Personally, I would laminate the back and simply seal all around the edges with varnish (if for no other reason than to toughen up the exposed MDF a little). If it's too late for that, I'd probably just give the whole thing one coat of varnish and see what happens. The way the fence is designed, I doubt it can warp very much anyway. I have MDF jigs in my shop that are still dead flat after a good number of years.

[Peter Quinn]

I've painted edges of templates with epoxy to seal and toughen them up a bit. For the back I'd just use the same hPL as the face for balance and scuff it up a bit so the fence plates don't slip.

[Jeff Monson]

Ken, I use shellac on mdf quite often, it is dry by the time you have your brush clean plus it seals it up nicely.

[Andrew Arndts]

I am just finishing my version of Norm Abrams "Deluxe Router Station". The fence is made of 3/4" MDF. The working face of the fence is covered with a laminate. The bottom, back and sides are bare sanded MDF.

Can I apply a fast drying polyurethane to seal it without causing it to warp or become crooked?

I live in an arid climate. Our average annual precipitation is only 13".

Or should I just let it remain unsealed?


I just watched that on the NYW site last week. I don't think he did anything perhaps edge banding... It seems that from the experience of others you could do whatever you wish.
don't forget some picture of the completed project

[Erik Christensen]

I am too lazy for all that stuff - for my MDF RAS table top I used water based poly & a brush - 1st coat was quick & light but I had no problems - I should have tried the dilute glue trick on the edges as it took like 6 coats to get it to seal up... but it takes 15 minutes to dry so not a biggie - the edges are pretty tough with all the poly - been in use 3 years and looks new

[Peter Kuhlman]

I have use shellac and poly with no adverse issues with the MDF. They both really toughen up the MDF and help prevent dents, etc. No swelling, warping, "grain" raising. I prefer shellac as I love the stuff but I think any type finish you have laying around will be fine. I have never tried any form of water born sealer like diluted glue and probably wouldn't try it as water does really bad things to MDF.

[Joe Scharle]

I use drywall joint compound to seal the edges of MDF. Sand smooth and brush on shellac.

[Tom Esh]

...
Or should I just let it remain unsealed?

Definitely not. Even in a dry climate it'll gradually start turning into something like cardboard from the outside in.
I've had excellent results using shellac as a sealer, followed by poly for a more durable topcoat. MDF is going to soak up lots of whatever goes on first, so shellac being relatively inexpensive and fast-drying is a good choice. You could of course just go with poly to start, but it'll use more and may take quite a while to cure completely.

[Phil Thien]

I have use shellac and poly with no adverse issues with the MDF. They both really toughen up the MDF and help prevent dents, etc. No swelling, warping, "grain" raising. I prefer shellac as I love the stuff but I think any type finish you have laying around will be fine. I have never tried any form of water born sealer like diluted glue and probably wouldn't try it as water does really bad things to MDF.

When diluted 50-50 with water, the glue is about as thick as paint.

Some day just give it a shot on some scrap.

Try new things, you may find some things work better than you'd expect.

:)

(BTW, this method is widely used by people that paint MDF. I'm not the only guy out there doing this).

[johnny means]

I like my MDF fence faces to be as simple as possible. No laminate, no sealing just MDF cut to size and machined to accept whatever connector is used. That way I can use them as zero clearance inserts when needed, screw or staple things to them, mark all over them, cut them down for a fresh edge, then toss them when they get short. I would leave the edges as is.

+1 on glue sizing. I've used it for years with great results.

[Peter Kuhlman]

Ok guys you sold me on the idea! I will try the diluted glue next time. :)