MDF, Wipe On Poly, and a Deteriorating Finish

A few weekends ago I built a prototype of a low-slung entertainment center out of MDF. SWMBO wanted a visual of the size and shape before I commited time and funds to the actual item. Sort of like this, but with no drawers or doors on the prototype:
http://www.sauder.com/images/drawings/normal/Ent_Credenza_401111.jpg
Since building the real one is going to be my winter project, I figured I would finish the MDF with a couple of coats of wipe-on poly. Now, a few weeks later, I am beginning to see spots and areas where the finish is turning white and other areas where the surface is cloudy and rough.
I store CD's on one shelf and the finish under the CD's is white and rough. There's a rough, cloudy spot under the DVD player where it looks like something dripped on it, but there's no way that could have happened - it's *under* the player. One exterior side has spots that look like something splashed on it, but it's on a rug with the side facing a area where no liquids would have been present.
It looked fine (well, fine for an MDF entertainment center!) for a couple of weeks and then I noticed these imperfections in various areas. Any idea what might be happening?
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Sounds like the MDF was wet in spots.
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- Sounds like the MDF was wet in spots.-
I doubt it was wet...at least not obviously. It was in my dry basement for few days, then in my shop. Built, dusted and finished. No obvious signs of moisture...it was evenly colored.
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could your player be shedding oil from a cap that may be going bad? how about oil slung out of the motor driving the disk? if it were me, i'd open the bottom of the player to see if there's anything in there or any kind of wetness on the bottom plate.
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wrote:

Thought of that...brand new unit, no leaks. Besides, my CD's would have to be leaking also.
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Maybe Cocaroach poo. ?
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CD decks are sealed, no vents, only a drawer up front. Sheet metal cabinet? High humidity?
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=====>All of the above and one other thought! Could you have been playing some real tear-jerker cds?*G*
Leif
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=====>All of the above and one other thought! Could you have been playing some real tear-jerker cds?*G*
I did play Stevie Ray Vaughan's version of "The Sky Is Crying" recently, along with some cuts from Leon Redbone's "Up A Lazy River" and a Muddy Water's CD. Who knows...
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wrote:

Clearly the woodworking gods are angry that you attempted to make something of MDF semi-permanent.
MDF really really doesn't like getting wet. I'd be a nice explanation to think that it got a good soaking before you bought it, then the surface dried out to where you would think it was fine. But the stuff would just disintegrate on you when you tried to work with it if that happened. So I don't know. Maybe you had a breakin by a very skinny cat that likes music.
I'd test that finish on scraps of that MDF and on something else. that should answer some questions.
-Leuf
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DerbyDad03 wrote:

Here's what I've found sometimes happens. MDF is usually transported in a pack, on open tray trucks. (In OZ it is) The top and bottom sheets are regarded as cover sheets and are not sold by distributors as they sometimes have diesel, road grime etc. on them. I sometimes obtain these sheets for rough projects, for 5 or 10 dollars each and they seem to suffer the same as you are describing, even after a good sanding.
Perhaps your supplier reckoned the sheet looked good enough to sell?
Just speculating
regards John
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Your explanation sounds plausible, but I have some doubts. These were 2' x 4' sheets and the defects are showing on parts made from 3 different sheets. I bought 6 of the dozen or so sheets at the borg so the odds of getting 3 cover sheets seems pretty slim - but not impossible. I certainly didn't notice any stains on them, 'cuz I'm pretty picky.
BTW - the 2' by 4' sizing is at best a minimum. The sheets I bought ranged from 24.5'' to 25" x 48.5" to 49". I planned my prototype around the 2' x 4' sheets to minimize cuts. Guess how well that worked out?
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wrote:

Well they started out as full sheets and the borg cuts them in quarters. So it would make sense that 4 in a row in the stack would have the same problem. I'd have thought something like that would show up right away during finishing, but I confess to have never tried to finish MDF :) Last time I got 2x4 sheets they were all the same size though.
-Leuf
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-- Well they started out as full sheets and the borg cuts them in -- quarters... -- Last time I got 2x4 sheets they were all the same -- size though.
But were they 2x4? If they did start as full sheets and the borg cut them, then either your 2x4 sheets were just under 2x4 or the original sheet was over 4x8. In my case, all my sheets were well over 2x4, some as much as an inch over in both directions. Assuming none of the sheets in the rack were under 2x4 (and I didn't see any) then the original sheet must have been way over 4x8.
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<snip>

It's not unknown that MDF sheets are oversized in both directions...
Patriarch
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Since you're in the market for suggestions, I have a theory.
Perhaps your wipe-on poly hadn't finished outgassing (or even the MDF), and the CDs prevented proper ventilation to allow the evaporant (is that a word?) to properly boil off. Under your CD player, perhaps some heat radiated downwards and did some damage to the surface.
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-- Perhaps your wipe-on poly hadn't finished outgassing (or even the MDF)
That theory has always been in the back of my mind. I let the final coat dry for about 24 hours and then moved it to the living room and loaded it it up. You could smell the poly for a few days afterward. I could tell you the whole saga of my kid's HS graduation, the out-of- town guests, the new living room furniture, etc. but suffice it to say I was was rushed and had to get it set up AQAP.
I'll call that theory the winner. Thanks!
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If you'd _previously_ mentioned 24-hrs from finish to 'installation' and loading, practically *everybody* would have jumped on that.
Poly generally needs _at_least_ a week to 'mostly' cure. Figure 2-4 weeks for the process to be 'essentially' complete. The curing process never completely stops -- it just gets 'slower' the closer to complete things get.
What you're seeing under the CD player is almost assuredly the result of a 'heat' issue from the CD player.
"Under the CD's" sounds like a 'lack of fresh air with something sitting on it' problem. The surface will pick up 'texturing' from the stuff it is in extended contact with, _before_ being fully cured.
In addition, *most* MDF _itself_ has an 'outgassing' issue for an _extended_ period (e.g., 6-12 months or more) after manufacture. Somewhat erratic, but it happens more/faster when exposed to even medium heat.
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