Sanding MDF????

Hi folks Just got some cupboards made up for the kids room with large MDF doors that I wanted to finish in a clear Poly. Unfortunately they have some dark stains on them when they arrived. I tried White spirits that actually created its own stain:~.

It looks like if I want to go down the Poly road I will have to sand them back.

So is it possible to do this without them going fluffy? IE does MDF have like a coating that once you sand through it you're stuffed or can you use a fine grit with gay abandon?

Sanding changes the colour but I assume that the poly will give it a wet look anyway but I also assume that the entire door will have to be sanded to avoid patchiness, am I correct?


Any help will be appreciated.

Don't forget that MDF dust can cause cancer. Make sure you mask up.

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Originally Posted by lbgking

Don't forget that MDF dust can cause cancer. Make sure you mask up.

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View the avatar!:cool:

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Originally Posted by Bleedin Thumb

View the avatar!:cool:

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and here I thought you were special forces Bleedinthumb

It sands ok but very slightly furry.

After the first sealer coat the nibs usually come off ok

Sorry, but it's stuffed for poly. The surface is much denser than the guts, and any sanding at all will cause a blotchy finish. The deeper you go (I'm talking microns here) the furrier it gets.
I've stained and poly'd a bit of it before, and you've got to use boards with a decent surface with no marks. Once you take the hard surface off it, it looks like crap with anything but paint on it.

Thanks for the replies folks, what you describe is what I suspected John, I might go back to the cabinet shop and tell them the problem, I'm sure that they will replace them no worries as they're nice blokes and three of sheets of MDF comes in under $60 and they will be able to use the old sheets for hidden structural stuff anyway.

The 6 doors that I need would probably take them about 2 minutes on the panel saw.

I've been through a similar experience recoating an old (well not that old) mdf coffee table. I took off the top of the surface by sanding and then recoated with poly, then discovered that due to varying porosity some bits were much darker than the others, as I had obviously sanded some bits harder than others. I wiped it off, and have now sanded back the mdf completely with a ROS stepping up in grades to 400 or so. I have found in the past that hitting with a water based sanding sealer seals the mdf without darkening it. You can then finish the top with poly and not get that dark mdf look.

Cheers,

Tom

Tom the sanding sealer sounds like a good tip.

I have changed my mind this morning and want to paint the doors with a flat acrylic - perhaps with a rag finish then brush on a clear poly coat over the top to achieve a hard wearing, easy cleaning gloss finish.( I'm sick of using enamels)

This begs the question - does the poly react/dissolve with the acrylic paint??

I would let the paint dry for say 5 days first. Has anyone applied poly over acrylic paint?

Thanks in advance again.

Poly goes over acrylic ok but poly yellows over time.

Instead of poly you could use Cabots water based clear floor finish, it dries nice and hard but is water clear

Bob, the colours I want to go with are browns/yellows so the yellowing will be OK.

I was going to use the Cabots wipe on poly Gel as it makes the job very easy, you don't have to worry about drips and runs.

I will have a look at the waterbased clear though - it may be even easier to apply...is it? Drying time may be a factor as I will have to apply the finish outside and there are a lot of leaves flying around at present even after making a humpy out of a tarp they seem to get in.

Gidday Thumbbleed:D

I've had great results finishing MDF with U Beauts Blond Shelac and A few coats of u Minwax Wipe on poly over the top.

Might be worth while experimenting a bit to see how/if you could introduce the colors u like................

I've got best results sanding back (ROS) to about 400 grit n a quick run with 600 I've had no luck with brushes n the like getting my best results from wipe on finishes using a lint free cloth

MDF is foul to work with & ranks as my all time cant stand to work with material. The dust is unbearable n fine so make sure as suggested u suit up on all the PP Gear! I'd suggest using a respirator! this stuff is just bloody noxious!!!!!

Anywaz just a thought to add to the mix!

REgards Lou:rolleyes:

Here's a couple of pics of a cupboard unit that I built out of MDF ten years ago now. Excuse the mess, dust, and cobwebs. It's my sons room and he's a bit of a pig.
I stained and poly'd it, and it's stood the test of time very well, although it does have a few scratches in it now:

http://i7.photobucket.com/albums/y26...h_P9170086.jpg http://i7.photobucket.com/albums/y26...h_P9170084.jpg

I'm committed now. Bought some Dulux suede effects paint and some clear water based poly to cover.
I'll post a picture and a synopsis of my stuff ups. Might be a few days as I'm renovating the bathroom, laundry and two bedrooms at present (my wife is nesting)as well as the WIP kitchen that I have been given the ultimatum over........not to mention kids holidays and work.

I've formulated a theory why women do that nesting thing when they're pregnant.......

They know that shortly they will suffering some serious pain so they have a need to inflict it on their partners.:D