"Veneering" ply onto MDF?

Marcel, I suggest you do some serious research into vacuum pressing. A mattress bag will most likely not do the job. There are bags and vinyl designed to take the extreme pressures and stress. There are also certain types of glues that are most appropriate for veneering. The temperature is important to know and control. West Systems epoxy is best when dealing with varying cooler temperatures. Unibond 800 works excellent above 70 degrees. You veneer should not be thicker than 1/8”. It most likely will show cracking later on if thicker. 1/16” is just fine. Mdf is a very stable substrate for a laminated table. You also must use a similar thickness and hardness of material on the top and bottom of the MDF. Otherwise the board will come out warped and never be able to be flattened. Both sides must be laminated at the same time, for the same reason. A true vacuum unit should be used. They are designed to keep the vacuum at the desired pressure. There are some simpler vacuum pumps that can be used with a compressor. The compressor will run the entire time the vacuum is turned on. I use a 10 CFM industrial vacuum pump that is completely controllable. It turns on with a vacuum pressure gauge automatically. The simpler vacuum pumps that run with a standard compressor do not have this capability because of their design.
It is best to do some studying about vacuum systems before starting a project so as not to ruin a lot of material and glue.

-- Oldworld, Fair Oaks, Ca

Marcel,
I totally agree with John on this. Your idea of using a mattress bag and a simple vacuum for clamping a tabletop is…well…kinda funny to be honest. You need a LOT of pressure for vacuum pressing. Go to www.joewoodworkwer.com for all you need to know about veneering techniques.
Why not use solid maple for your tabletop? Veneers are used when you want to use crazy expensive exotic woods or when you want a book matched effect. For a plain maple top, I’d use solid maple. Less trouble, much sturdier. About using MDF for veneering: Birch plywood is a better choice because MDF and also pressed wood panels will swell if exposed to humidity (like if you get a spill…which is very likely to happen on a tabletop).

It’s expensive to get into vacuum pressing. As said above MDF is great for veneering, very stable, but can get heavy. Another way to veneer is to use mdf as cauls and use clamps and pressing bars spaced about every 4-5 inches to give you the pressure. I wouldn’t use any kind of ply. what would be the point, it would be easier and cheaper to use just commercial veneer over the MDF, or a solid top. Maple is pretty cheap, $2-5 a bf depending on where you get it.
Or if you want to use ply, it’s easy to get a 3/4 maple ply.

-- Joey, Magee, Ms http://woodnwaresms.com

I’ve never done this before for a table top but you could try using contact cement with a piece of 1/4” plywood for the veneer. I would also seal the underside of the mdf to seal out as much moisture as possible. You’ll need a really flat surface for assembly.

-- Dave Herron, Boise, ID -- How hard can it be? It's only wood!