Painting MDF?

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If you have seen any of my posts before, you'll know that I usually think I know it all but apparently I don't and now I have a few questions. My domain consists of solid wood and the ocassional ply for all my finished goods type production. I use MDF all the time for fixtures, etc. but have never incorporated it in a finished piece.
I am building a fireplace surround and mantel for my new home. It seems best to build some of the parts from MDF. It will be painted.
Q's.
1. Is MDF paintable? It ssems the moisture will screw it up. Is there some priming method required?
2. I seem to recall a variant that has a paper coating. Is that required?
3. What about edge prep if you are using the papered version.
4. I will have a scenario where I want to use butt joints. I want to sort of puttey the end grain (or edge) and fill the gap of the butt joint and sand it out smooth so it won't show once painted. Could I just use like sheet rock taping compound or is there something better?
BW
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Yes, mdf takes paint very well - at least on what I will call the "face grain".
On the "edge grain", yes, you should prime it. ordinary joint compound and a light sanding works well.

You're thinking of "MDO". MDO is NOT the same as MDF. MDF is "Medium Density Fiberboard". MDO is "Medium Density Overlay".
Originally developed for sign painting and concrete formwork industries, MDO is also used for other applications as a paintable surface.
It is a plywood product, with a paper face overlaid on the plywood substrate with an exterior grade (usually) phenolic adhesive. MDO is designed to have a smooth paint receptive surface. One or both faces can be faced with paper, and is found in 4' x 8' sheets from 3/8" to over an inch in thickness. MDO is used for short term signs. MDO is not recommended for permanent sign faces because it is susceptible to Warping, Water log, Mold Etc.

MDO can be edge-banded or filled. It is plywood.

However you join MDF, take care with the fasteners. Use "Spax" screws or the like. You need a coarse thread with MDF.
Also, be aware that MDF likes to "mushroom", so use of countersinks on BOTH sides is recommended.
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I have a customer that moves every 5 years or so and insists on redoing the kitchen each time. He is sold on MDF flat panel doors, which I cut out and drill for Euro hinges, he spray paints the panels in his garage. He typically lives in $500,000 homes so I don't get the attraction to this type of door but it has the look that he is going after. That said, these MDF doors are the only thing he spray paints and I'll have to say that his results are pretty darn good. He does use an oil based primer and paint and lightly sands the primed surface before applying the final paint coats. No sanding of edges before the primer goes on.
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I apply Kilz or Zinsser primer and then sand it smooth.

Work wallboard compound or wood filler into the edges. They'll soak up lots of paint if you don't. It's best if you don't have exposed MDF edges. I would put edge trim there.

Use biscuits, splines, or backers to keep the surfaces aligned. Fill the crack with wood filler and sand it smooth, then do the prime and paint.
--
Steve Bell
New Life Home Improvement
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I used sanding sealer the other day to seal up MDF. It seemed to work well. Greg
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What we have done, with good success, is to fill the edges of MDF and plywood, which are very absorbent, with lacquer-based automotive putty. One brand we use is called Green Stuff. It is the last filler/ putty automotive guys use before they spray on the primer. It dries quickly and sands beautifully.
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Now that's interesting. I have been using Durham's Rockhard for years on that stuff. It dries fast, sands very well, but on occasion there is an adhesion problem with small fills (like large nail holes) in materials like MDF that tend to be hard and dusty.
Any adhesion problems? How fast does say, and 1/8" diameter take to dry before sanding?
Can you put latex coatings on it as well as solvent based?
Have a manufacturer handy?
I am seeing and using more and more of this stuff in different grades, and frankly, as poor as the wood is that I have been purchasing for some time I am getting to the point I prefer it. I prefer the end product, but don't prefer working with it.
Man, I miss wood. If you had asked me 30 years ago if I thought we would be using modified cardboard for baseboards, finger jointed nastiness from Chile for trims, sheets of termite leavings instead of doug fir (which I thought was crap at the time) I never, ever would have believed it.
Robert
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wrote:

The stuff we buy, is green..and called Green Stuff... I keed you not. I also seen this around the shop, I think it is the same kind. http://tinyurl.com/8yjl2l
I'll look on the toob when I see it.
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I would never have thought about using that stuff on wood. I would love to find a super fast dry filler. That stuff doesn't look like it would be good for trim nail holes, though.
I am looking forward to seeing what you actually have. I <love> to have some trick stuff to make my work easier and faster in the toolbox.
Robert
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snipped-for-privacy@aol.com wrote:

The downside to using lacquer putty is that lacquer shrinks. It needs to cure *completely* before you can sand it flush and not expect it to shrink further into whatever recess you're putting it. Then, if you're topcoating it with lacquer all bets are off because the solvent in the topcoat will turn around and soften the putty all over again. Using it to coat the "end grain" of MDF seems pretty safe though since you're not really filling any "holes" to speak of.
--
Free bad advice available here.
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wrote:

When my guys were assembling cabinets, melamine on 5/8" PB, sometimes they'd pop a nail through the side and make a larger hole. We had this can, with white body putty. It was made by Sikkens. We'd whip up a wee bit with a dab of hardener and 20 minutes later flush the repair with a sharp chisel. Fast, white, and tough.
The lacquer putty was never used for larger problems, because it does shrink when it dries. I am not aware of any problems with the stuff softening with actual lacquer after it had set up.
I do not know where the cut-off is in terms of hole size. It would surely be successful in a 18ga nail hole.
I worked on the restoration of an important house in Rochester NY. The painter there used window putty to fill nail holes and just painted over it. No waiting or sanding. Worked great. And this guy knew his stuff.
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Yes, but why would you want to? At worst, three coats of clear poly. Best would be a shellac and French polish. If you are going to take the time to make a nice project, don't cover up and hide the material. I use 8d common nails too and the contrast with the heads is a beauty to behold.
Given your sense of aesthetics, I don't think we will ever see you doing one of the Designer shows on cable TV.
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Ed Pawlowski wrote:

I've used KILZ on a whole sheet before cutting. Rolled on one coat and a light sanding-- it looked like melamine.
--

-MIKE-

"Playing is not something I do at night, it's my function in life"
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Not sure I follow. Was this some kind of jab at my work?

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On Fri, 16 Jan 2009 21:36:21 -0800 (PST), "SonomaProducts.com"

...maybe, maybe not...heh...OTOH, I've built a rather large hearth out of MDF, with wood plant-on trim, mitered the corners and filled where appropriate with Bondo...used poplar for the mantle and hired a boat painter to apply a two-part varnish. Been 10 years and still standing tall...MDF paints up great!
cg
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Surely you must have some sense of humor. Have a coffee and re-read. You may also want to dig up some old posts about finishing cherry.
wrote in message

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I thought the only appropriate finish for Cherry was semi gloss latex paint. Maybe I'm confused.
I am covering an existing brick fireplace and mantle so I am a bit constrained. I also have a home that is not so upscale so I want to bring in some class but not so high end that it is over done. I am using some classic influences and holding the stacked molding down to a minimum. I am using an MDF backfield to be CNC'd by my buddy to match the shape of an archway I am constructing betewwn the living room and kitchen to tie the things together. Here is a link to my current design.
http://www.sonomaproducts.com/images/fireplace.jpg

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Hey.... I like that! I'll bet painted out a classic ivory or white it would look great. Did you do the design work as well?
Robert
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Thanks. Yes my desing, I collected many images of classic designs and then drummed up my own designed over the existing brick. Yes, white. The room walls will be some ivory or slightly darker. The bicks, unfortunately, are already painted white so I think I will try doing them the same color as the room walls. If I thought it could be done I might try sand blasting back to red brick. I might try it first on a portion that will be covered. Maybe even just sanding it off a bit for a distressed look.
wrote:

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I thought the only appropriate finish for Cherry was semi gloss latex paint. Maybe I'm confused.
I +++++++++++++++++++++++++++++
Exactly. The opposite is MDF finished with the grain showing.
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