Should I seal the inside of this table top?

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Forum topic by mummykicks posted 09-16-2013 03:01 PM 951 views 0 times favorited 2 replies Add to Favorites Watch
View mummykicks's profile


85 posts in 1226 days

09-16-2013 03:01 PM

I’m making a drafting/art table that has a blackboard surface (.03” steel with a coating) on a piece of 3/4” mdf, looks like this:

It’s 34 1-/2×28 1/2. That will basically be a piano hinge on one end and two supports on the edges to provide the tilt. They will be contacting on the left and right edge in the picture below, which is why the horizontal ribs are closer together there.
Since it’s going to be largely unsupported I don’t want it to sag over time, so I’m making it a sandwich with plywood ribs inside like this:

The bottom will be a piece of 1/4” ply that gets glued/screwed on.
I know plywood is supposed to be stable, and so is mdf, and yet I have warped examples of both in my garage.
I also know that sealing only one side can cause warping as well.
So the question is, do I seal the inside of this with shellac before I close it up?
The top face of the mdf is effectively sealed by the blackboard (like a piece of laminate).
I’m leaning towards not at the moment…

2 replies so far

View a1Jim's profile


115177 posts in 3000 days

#1 posted 09-16-2013 03:17 PM

Your right it’s always best to finish both sides of furniture but it’s less critical when you using plywood,this table looks like it might be hard to finish without getting finish on the area you will apply your glue .

-- Custom furniture

View GrandpaLen's profile


1643 posts in 1696 days

#2 posted 09-16-2013 04:21 PM

Your design is in essence a torsion box, as I’m sure was your intent, and if you run a bead of glue the length of each of the inner webs there should be no twisting or warpage. With that said, personally I would seal the edges of the MDF to keep them from drawing moisture and swelling out of shape, even if you plan on trimming the edges with solid wood.

I used Gorilla Glue for the torsion box top for my Router Table, it’s a polyurethane glue an is incredibly strong. Read and follow directions on the label about mistng one surface with water.

Best Regards. – Grandpa Len.

Work Safely and have Fun.

Edit; This Gorilla Glue foams and expands a little during the curing process so you will need to clamp the edges fairly well and use some weights across the length and width of the MDF top and you will be in good shape.

-- Mother Nature should be proud of what you've done with her tree. - Len ...just north of a stone's throw from the oHIo, river that is, in So. Indiana.

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