Official Luthiers Forum!

Hi everyone,

I just successfully routing my first two radius discs out of MDF. I read somewhere that those discs can warp if you don’t put some kind of a finish on them or glue them to another full thickness piece of MDF. Any insight on what is the best way to do this. Should I put some kind of finish on them or glue them to another backer board of MDF. Thanks so much for your help.

I glued mine to another 3/4” mdf. I didn’t put any finish on them though. They haven’t warped. I have one that I left single without the backer and it is warped.

I don’t have any answer to the question but this post is in the 2013 new builder’s challenge forum which no one is still looking at. Only people who use the “view new posts” feature will ever see it. . .

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Absolutely apply finish. It will protect it from moisture and glue. I use shellac or Zinnser SealCoat.

How many coats Barry. That MDF really sucks up the finish!

I did the same thing Barry did. I clamped the dish and the backer to the top of my table saw to keep them flat. Glued and screwed and left clamped for a couple days for the moisture from the glue to dissipate.

Many coats of Zinnser Seal Coat, I'm thinking 7 or 8, maybe more. I wanted a smooth surface to be able to peel off PSA adhesive on sand paper.

If I did it again I'd screw and glue two radius dishes together back to back and save some MDF and shop space.


As a side note-
I used Zinnser on my MDF bending molds after testing every kind of finish in my shop with heat of bending blankets. I was surprised to discover that nearly every other finish turned to a sticky goo in the presence of heat and shellac held up the best. I put fewer coats on the mold, but enough to seal the MDF.

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Brian R, Wood Mechanic
N8ZED

I applied enough coats until I started to get a build up that I could sand smooth. I don't remember how many coats but it was probably 4 or 5.