Problem painting MDF


Hi everyone, I am working on a large (24in x 34in ) shadow box that needs to be painted gloss black. It needs to be done cheaply so I used some MDF that I had laying around. I'm using cheapo spray paint from WalMart and am having trouble with the rounded edges not taking the paint. The flat surfaces are fine but where I used a round over bit the MDF is behaving like end grain-it's just sucking up the paint. I shot a primer coat and three top coats and still cannot get a nice finish. Before painting I also sanded to 220 grit so it should be smooth enough. Can anyone shed light on this problem? Thanks, Gene
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Think sponge.
You need to seal the porosity. I use lacquer-based automotive finishing putty.
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Gene T wrote:

The spray paints (Pressure Packs) are just to thin for MDF edges. Get out a brush and give them a coat of acrylic Black or undercoat-sealer. Sand back nice and smooth and then spray. All should be OK. Regards John
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An article in FWW and/or FHB showed sealing the cut edges of MDF with thinned drywall taping compound. Fills, smooths and pretty much seals the edges. Use before priming.
Or so I'm told. I use very little of the stuff, personally.
Patriarch
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When ever I finish MDF with paint I prime the hell out of it. I use Krylon spray primer and lay it on until it looks like it will run, then I let it dry, and put on one or two more heavy coats, with a light sanding in between. Keep laying on the primer until it quits absorbing into the MDF. Greg
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Thanks for all of the useful info guys. I didn't realize how porous MDF was. Now I wish I had used polar instead!!! Gene

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Now I wish I had used polar instead!!!

...working with that stuff can give you a disorder. . . . . . . . . . . . . . . . . I seldom make fun of a typo..sorry 'bout that.
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You can also buy special primer for MDF too :) Most big box tool stores should have it.
-- Regards,
Dean Bielanowski Editor, Online Tool Reviews http://www.onlinetoolreviews.com Complete our tool survey, Win $200! ------------------------------------------------------------ Latest 6 Reviews: - Sherwood OS-100 Oscillating Spindle Sander - Porter Cable NS150A Narrow Crown Stapler - Book: Popular Mechanics Shelving & Storage - Betterley Tru-Cut Insert System - Digital Calipers & Height Gauge - Delta SS250 Scroll Saw (Review Updated) ------------------------------------------------------------
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to
that
like
sanded
Unfortunately I work with MDF all too often, I find no joy out in working with it whatsoever.
I make a light wash of PVA and water and "pre-prime" (hey, did I just invent a new term?) the MDF edges with that, let it dry and then prime & paint normally. Occasionally I may also need to do some filling but I do that after priming and usually add a second coat of primer over the filler.
It works for me, anyway.
- Andy
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I used shellac for this purpose and found that it did a good job of sealing the surface. After each coat (did several) I sanded the surface to make it smooth. Has anyone tried shellac as a primer? Have you found that the other methods mentioned did a better job?
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I have used it ith success.
I like shellac for it's quick-drying, and it "sandability".
-Steve

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Oh yes, I forgot that you can add some sandarac (about 10% or so) to shellac and it makes it less "gummy" and more friable so it sands a lot better. Sandarac is a gum resin that can be purchased from some finishing suppliers.
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I haven't used plain shellac, but I have used Zinsser BIN primer, which is a shellac based primer with white pigment added. It worked very well for me.
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Larry Wasserman Baltimore, Maryland
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Andy McArdle wrote:

A.K.A. "Glue Size".
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invent
Ah. Thank you. Terminology's one of the problems with being self-taught...
- Andy
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Andy McArdle wrote:

I know the feeling.
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Lessee... glue sizing isn't about wallpaper, tenons go in mortises instead of chucks, and if a plane flies you're doing something wrong? Next I'll be told a jig is not when you hit your thumb and lathing doesn't involve plaster.
No. No, I'm not getting the hang of this. I'm going back to pointy sticks.
- Andy
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