Sealing Glued/Screwed MDF Between Sheets?

[mason winston]

Thanks for reading! :)

I'm going to start work on the top of my reloading/general use workbench this afternoon, and I have a few questions about working w/MDF. First, I'll give you some (hopefully) useful info:

- I'll be using two sheets of 3/4" MDF, totaling 1 1/2" thick, glued and screwed as my worktop. Both sheets will measure 24" x 72".

- The MDF worktop will "drop in" to the bench frame, so that the top of the MDF will be flush w/the top of the 2" x 6" frame.

- The bottom sheet of MDF will be screwed to 2" x 4" support beams. The support beams will be glued/screwed to the 2" x 6" frame, so the entire top of the bench will be connected and solid.

- I'll be using Titebond 3 for all glued joints, including gluing the two sheets of MDF together.

Here are my questions:

- With regard to the MDF surfaces to be glued/screwed together, would it be a good idea to lightly sand and then seal with Zinsser BIN clear de-waxed shellac (or original Zinsser BIN white) primer before gluing?

*For clarity, I am asking whether or not I should prime/seal the inner "sandwiched" MDF surfaces prior to gluing.*

- Is there any sense in applying two coats of white Zinsser BIN, and then applying one or two coats of clear Zinsser BIN as a sort of sealer/protectant/clear coat for the worktop? I plan on leaving the top of the work surface white. Easier to see/find little springs and pins from guns that way.

-When I searched for ideas about finishing MDF, I'd say 90% of you all specifically mentioned Zinsser BIN. Why BIN and not original, oil-based KILZ? Is it the shellac base in the BIN?

[Prashun Patel]

You might know this, but just for clarity: Zinsser BIN primer is a paint primer. Zinsser Sealcoat is dewaxed shellac. The BIN primer has some shellac in it. BIN is usually recommended when MDF is going to be painted with a protective topcoat. The white BIN itself will be a magnet for dirt and stains without a top. I've never tried KILZ on MDF but the BIN seems to seal it quickly.

You should not prime or seal the MDF prior to gluing. Wood glue will adhere to raw mdf just fine - especially when reinforced with screws. Remember to pilot drill yr screw holes. Some people even countersink the hole on the bottom sheet where the pieces mate to provide for displacement.

Beware that most people around here don't favor a film forming topcoat. They can get ratty looking very quickly. Many prefer an oil/varnish blend or Boiled Linseed oil and or wax.

[Conrad Fiore]

Mason,
Glad to see you are going with the MDF, it will add the weight required for a good reloading bench.
As Prashun mentioned, do not seal the MDF before gluing, glue to raw surfaces only. He also mentioned relieving the top side of the bottom hole. This is a good idea but can only be done if you drill all your holes before the glue-up. Test this by drilling a pilot in two pieces of scrap MDF and screwing them together. You will see that as the screw point exits the lower piece of MDF and enters the top sheet, the screw threads will pull MDF out from the top sheet and will cause a wad of MDF to form between the two sheets and open a gap, rather than close the two pieces together. As previously posted, you can correct this by counter sinking the top of the hole on the bottom sheet and this will give a place for the wad of MDF to seat into.

Have you thought about laminating a piece of white Formica to the top of the MDF. You could paint it white and then put a coat of clear over that to prevent blocking and make for a more durable finish, but I think a sheet of Formica would be a ton better than any painted or finished surface.

Here is a sketch of the MDF drilling and counter sinking.

[Howard Acheson]

Let me add the MDF does not work well as the working surface for a bench. It is easily dinged and damaged. It's not a very strong material as far as resisting abrasion or impacts.

What most folks due is add a top layer of 1/4" fiberboard (masonite) to you laminations of MDF. The fiberboard is much more ding and abrasion resistant. It is also easier to finish. It can be replaced if it gets too banged up if a removable method is used to mount it on top of the MDF. Some use double faced tape or dabs of hot glue or silicone adhesive.

[Jerome Hanby]

I concur, no finish on the faces to be glued, and use tempered hardboard as the working surface. If you are banding the MDF, you can take the hardboard into account and make it very neat. If you are worried about the MDF out-gassing formaldehyde, then shellac makes a good sealer...

[mason winston]

Prashun & everyone else: I am a total newbie w/regard to woodworking, so if at any time you feel that clarifying something might be a good idea, it probably is! I'll eat up any and all extra bits of info you're kind enough to pass along. Thank you! :)

Ok, so primed/sealed MDF probably won't be a durable enough work surface for me. What's the general thought on lacquer? Would a coat (or three) of lacquer harden up the MDF surface enough to be a satisfactory worktop?

I'm not against using a 1/4" hardwood veneer on top of the MDF by any means. I'm sure it's just my own inexperience showing, but I'm really liking the idea of a top that's a nice, flat white primer covered with a clear, hardening solution. That's where the lacquer comes in.

I hope you all know that I actually am listening to you, a flush-fit MDF top is proof of that. And I really, really appreciate the time you take to share your experience and knowledge w/me. :D

[Prashun Patel]

If you're like me, you'll go through several benches as your need and appreciation morphs over time.

So, I say, if you want it white with a filmforming topcoat, then go for it. If that doesn't work for you, it's easy enough to redo or cover or whatever. You'll discover for yourself the pros and cons of what yr going to do. The stakes are low, and the learning is valuable.

[mason winston]

Out of curiosity, where should I look if I wanted to find Formica veneers? I looked, though honestly not very hard, at HD yesterday and didn't find any. I did see plenty of completed countertops w/Formica already bonded to a manufactured wood slab.

Also, how easy (or difficult) is it to drill precise holes in a Formica sheet? I'll poke around the 'Net to find techniques that help avoid unwanted damage to the material surrounding the drilled hole, but I'm also curious what tricks/tips you guys may have to offer.

:)

ETA: I have no experience working w/Formica. That's all.

[Howard Acheson]

>>> Would a coat (or three) of lacquer harden up the MDF surface enough to be a satisfactory worktop?

There is no finish that will "harden" a wood or composition material short of 1/4" thick layer of pour-on epoxy.

[Henry Ambrose]

This is a good place to use polyurethane. MDF will suck it up and it will make a tough work surface after about three coats. I have some machine stands done this way and its pretty tough. You can finish the whole thing with poly. It'll give you a good tough finish for a reloading bench or other utility/work type furniture.

When gluing mdf I use a roller to cover both surfaces, let it soak a bit then add one more coat and drop the sheets together. You can also brush the glue on but be sure to get an even amount applied all over. Put some paint cans on top to weight it and come back to it in a couple of hours. Use screws if you're in a hurry to move the assembly along.

Depending on your budget and appetite for spending a little more time on the project, I'd consider adding another layer of material on top of the whole bench. You can have a smooth top and no little cracks or grooves for things to get caught in. What ever structure you started with is still there, you'll just need longer bolts to bolt the press down. If you make it flat enough you can stick plastic laminate on top and use the 2x6 framing as the edging. Do you have a router? You'll need one for plastic laminate work.

[Conrad Fiore]

Mason,
I just bought a sheet of Formica at HD. At most locations, you have to ask the sales person in the Kitchen Dept. and they will order it for you and will take about 4 days to get in. Cost for Grade 10 (Std. Grade) is about $1.65 a sf.
It is best to drill with a brad point bit or you can start the hole with a Forstner bit to get through the laminate and then finish with a std. drill if you don’t have the brad points.
As Mr. Patel wrote, this is your first bench and without much in the way of investment in cost of materials, you can just do what you originally posted and make it out of laminated MDF, paint it white and put a coat or two of poly over the top of that. For the reloading/gun repair surface it will last indefinitely. If it gets too beat up as a work bench, you can make another top with another surface material at some later date.

[Chris Padilla]

Definately go the formica route...it will GREATLY extend the life of any MDF worksurface. You'll need contact cement for this and a j-roller.

[Prashun Patel]

IMHO, this is making a mountain out of a molehill. With Formica you gotta deal with cementing it, cutting it, trimming the edges.

This is your first bench. Just do it originally how you intended and learn on it. Benches are personal. If you search these threads, you might get the false impression that everyone has a 3" thick solid maple block on top of a 96" x 36" frame with front vises, tail vises, miami vices, square dogs, round dogs, hot dogs in every place possible.

Personally, I learned about what I want/need in a bench by slapping one together out of 2x4's and a 3/4" particle board top that was shellacked on half and polyu'd on the other half (because that's what I had and knew no better). It served well and I was a happy person then!!!

[Chris Padilla]

IMHO, this is making a mountain out of a molehill. With Formica you gotta deal with cementing it, cutting it, trimming the edges.

...but making a mountain out of a molehill is one way to learn. :)

Like anything, having the right tool makes things easy...like trimming formica flush (Bosch COLT). :D