Vacuum Plenum - MDF problems

[gerald_martin]

Hello Shopbotters:

I cut my first vacuum plenum board (4x8) from MDF Saturday, and sealed it with a waterproofing sealer for wood/masonry. I was very disappointed with the result - it seemed that the MDF soaked up enormous quantities of the thin liquid and swelled up...

In retrospect I think maybe I should have used a urethane sanding sealer type product? But, after this experience...

Is it really sensible to use MDF? Is there some type of plastic or PVC material that would be air tight, require no sealing, and be more rigid than MDF? Gary Campbell you did something like this, right?

I would rather abandon the MDF and do something else now if that is what needs to be done to make a reasonably permanent plenum board. But, if I just used faulty technique, I'm open to advice.

Gerald

[thewoodcrafter]

Sorry to here the problem.
Why don't you just throw a second sheet of plywood on the table and cut that. Some Baltic birch.

[gerald_martin]

Interesting idea Roger. I used Baltic Birch for the support board on my table - the stuff machines pretty nicely but I had trouble with long sections of the surface ply coming loose where I drilled by carriage bolt holes thru. Would I have to seal the Baltic Birch too?

Gerald

[beacon14]

If you use Baltic Birch I don't believe you would have to seal it. Very dense wood and many layers of glue make it all but non-porous. I'd still seal around the outside edges just for good measure but I don't think you would loose any air through the bottom of the plenum with two layers of BB ply glued together.

PVC is also a good choice if you can figure out how to bond it to the plywood base. A few folks have used a single layer of PVC instead of the plywood and MDF layers. If so I'd want to use 1" minimum thickness for stiffness.

[knight_toolworks]

I tired a poly finish to seal the mdf big mistake smelled for months. the fastest thing to sue is some thinned yellow glue brush it on and it is sealed in one coat and dries pretty fast too.

[gerald_martin]

Thanks for the feedback everyone...sounds like I made a mistake. Other ideas, anyone?

Gerald

[sawkerf]

Gerald,

I've had mymdf plenum for 4 years with no problems at all. two or three good coats of polyurethane and your set. Kip

[cip]

Gerald, I used a product call Extira, it is water proof MDF. It's made with resin glue instead of Formeldahide glue. It works just great and it isn't much more then regular MDF. It needs no sealing and is air tight.

Mike

[cabnet636]

i used an acrylic clear on mdf and it worked well,

i was considering using phenolic on my next one as i found some in fla for under 500.00 per sheet but i did not think of exteria, thanks!!

jim

[gerald_martin]

I like the Extira idea, is there any downside to this? No sealing would be a real advantage.

Gerald