I had read somewhere that a urethane was 'required' to prevent
formaldehyde release from MDF. I believe the MDF does release
formaldehyde just sitting, because, as an example, if I buy a sheet
of MDF and put it in my downstairs workshop I can smell it the next day
as soon as I open my front door. I find the odor really powerfull.
Although some brands don't seem quite as smelly.

Thanks for your reply.

Zander