air flow through unsealed MDF under vacuum

[drcrash]

Does anybody know how much air flows through unsealed MDF under vacuum?

My understanding is that it's reasonably high; MDF is pretty porous stuff.

Anybody have an actual data point? (Say, a tank of a certain size filling up at a certain rate just from flow through the bottom of an MDF platen, or vacuum-clamping surface, of a certain size?)

I'm also wondering what's the easiest, cheapest way to seal MDF pretty well. My understanding is that a lot of paints and varnishes don't do a good job---as they dry, they become microscopically porous where the solvent evaporates away. Even a bunch of coats won't help as much as you think. (I've used polyester & epoxy resins, but that's a bit of a pain in the ass, and a little more costly than I'd like.)

[mxtras]

I have over 10 years experience working with an MDF vacuum table but I have no data points. I do know it flows better when it is skim cut. I know this because I have a commercial router with a gauge in the chamber and when the 1" waste board is not cut, the gauge will read a higher vacuum pressure than when both sides of the board are skimmed. Higher pressure in the chamber when the table is wide open indicates a restricted surface. In theory, a guage below an open table with no restrictions will read next to nothing for vacuum pressure. To me, this is the most desirable condition. Effective distribution is also important.

The best way I have found to seal unused surfaces is with clear plastic packing tape. It's cheap and very effective.

Scott

[martinw]

I'm also wondering what's the easiest, cheapest way to seal MDF pretty well. My understanding is that a lot of paints and varnishes don't do a good job---as they dry, they become microscopically porous where the solvent evaporates away. Even a bunch of coats won't help as much as you think. (I've used polyester & epoxy resins, but that's a bit of a pain in the ass, and a little more costly than I'd like.)

Dear drcrash,

A really easy way to seal MDF is to mix three parts of water to one part of PVA glue. Just paint on a few coats. You can then paint on some oil based paint if you want (you may not need to, but I did). I used this PVA and oil based paint approach on a "vacuum bag" set up which was about 5 ft by 10 feet on plan. The bottom of the "bag" was a sheet of 18mm MDF. A PVC cover-sheet went on top of the MDF and was sealed to the MDF by rubber gaskets. Without the PVA and paint, I just couldn't get any vacuum at all. With it, it worked a treat.

Best wishes

Martin

[kayaker43]

I've been meaning to set up some tests, but never find the time. I figure if I stall long enough someone else may do it

You can see the denser surface layers on a fresh cut edge, it looks like maybe 1/16 deep? The center looks less dense. It varies between brands too, some are darker, harder and heavier.

There are higher densities, sometimes called harboard or tempered hardboard but I've only seen them in thinner sheets. One brand name is masonite. This density may not flow much air at all, need to do tests? McMaster Carr sells the same thing only much harder, its hard like a rock and nearly black, I've used it for making router templates.

There's bound to be some common goop that will seal ordinary MDF, or even particle board for that matter,.. mobile home roof coating or something?

[coonhunter]

Duct Tape.

[drcrash]

Thanks all for the suggestions and useful info.

I've got a half-made new platen, and when I get around to finishing it, I may get a chance to do an experiment.

I'm thinking of testing it unsealed, then sealing the bottom (back side) with a couple of coats of sandable primer to see if that has much effect, and if not, some diluted glue. (I'm sure duct tape would work, but it eventually creeps and gets gummy, and maybe peels.)

I'm wondering if the sandable primer might be good enough, if it's a reasonably high-solids product. (As a sandable primer should be.)

[kayaker43]

I'm pretty sure primers dry porous? As a kid I would start some bodywork on my car and just prime the bare metal till I could finish it. It usually shows rust within a week without a top coat. High solids but I think they're not as fine as regular paint pigment?

Try gloss latex house paint? I peeled a patch off a waxed metal plate and it was like a thick rubbery plastic sheet.

[drcrash]

I'm pretty sure primers dry porous? As a kid I would start some bodywork on my car and just prime the bare metal till I could finish it. It usually shows rust within a week without a top coat.

Yeah, I got to thinking about that. (That's why my posting about paint that hot plastic won't stick to had a preference for smooth, maybe gloss.)

High solids but I think they're not as fine as regular paint pigment?

Yes... I suspect they dry porous so that whatever you put on them can insinuate itself into the pores and make a good bond.

I don't think it's the pigment that's different, though... and now that I think about it, I don't know if there's not much difference between gloss and matte paint in terms of actual smoothness.

As I understand it the difference between gloss and matte paint is the size of the pigment particles. The ones in gloss paint are so tiny---comparable to a wavelength of light---that light reflects off of them as though they were a solid sheet. The ones in matte paint are really microscopically tiny, too, but not quite that tiny, and light waves bounce off them in random directions. I suspect the difference in the scale of bumpiness doesn't matter for vacuum sealing, because the binder smooths over them at that scale and it's just light that "cares." (The light goes through the transparent binder and is or isn't scattered.)

I could be wrong, though; the bumps might affect the surface and be big enough to make channels for seepage.

[Geof]

I think practically all paints dry porous. The explanation for this that I got from my Dad who was a painting contractor was that paint has to be porous so that water vapor can pass through. If it is not porous, or not porous enough, that is when you get separation of the paint film and bubbling.

[automizer]

I have run large table routers 5' by 10' with MDF spoil board , and in a pinch for a complex shape a simple shop garbage bag works really well. They worst part of this design is as soon as your tool breaks threw your product you have now made a leak, Unless your blower motor is huge to compensate for these leaks it will become a nightmare.

[drcrash]

I think practically all paints dry porous. The explanation for this that I got from my Dad who was a painting contractor was that paint has to be porous so that water vapor can pass through. If it is not porous, or not porous enough, that is when you get separation of the paint film and bubbling.

That sounds familiar, now that you mention it. I read up on paints a few years ago, and I probably knew that.

I wonder if auto paints are the same way? I could imagine not, on the assumption that metal or fiberglass doesn't have any significant moisture content if you paint it when it's dry, and you really don't want moisture getting in if it's steel... but I could imagine it the other way, too... if a little moisture is in there, you want it to get out rather than rusting the metal under the paint.

I've been using an epoxy coating that's mostly sold for sealing furniture. I can imagine that even that is designed to be porous, with just enough solvents that evaporate out after it's set to leave microscopic air channels for wood to breathe. Any sealer designed for wood seems likely to be the same way, unless its designed for continuous immersion in water.

(And even that's not a gimme. It might be designed so that gangs of hand-holding water molecules can't get in, but individual water vapor molecules can get out, and loose gas molecules can get in or out, vaguely like Gore-Tex.)

This is ridiculous. I could always laminate a sheet of plastic or foil to the bottom of a platen, but there's got to be something you can just spray on that makes a reasonably air-tight coating.

My guess is that just primer will slow the air inflitration, by clogging up the big pores in MDF, and that adding a coat of regular will slow it more, by clogging up the medium-sized pores in primer. Then there'll be the tiny pores in housepaint left.

I can imagine that'd cut air inflitration by 99 percent, with each step blocking 90 percent of the air. Or maybe just by 50 percent, with each step not doing much---just replacing big pores with smaller pores, but more of them.

[drcrash]

This is ridiculous. I could always laminate a sheet of plastic or foil to the bottom of a platen, but there's got to be something you can just spray on that makes a reasonably air-tight coating.

Hmmm... now I'm wondering if something like Armor All would do the trick. It's supposed to seal & protect plastics from oxidation (as well as UV). Paint is basically porous plastic that needs sealing to keep air out.

So prime, paint, and coat with Armor All?