mdf trim

using mdf sheet material to do my 3/4 returns and trim on my vinyl windows. end product will be painted white
Just wondering if it is necessary to prime and paint all sides before fitting the returns and trim, or prime only?
Also, what do I fill the nail holes with?
I realize if painted prior to fitting, they'll need hit with another bit of paint, which brings me to another question.. is it worth using a small roller to apply the primer & paint or just use a brush?
Thanks again.
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Yes. Spackle. Roller.
You're welcome.
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I know you didn't ask and you won't want to be told this but really you need to burn the MDF and use wood or plastic. Even MR MDF is messed up by water be it condensation, roof leaks, spills, rising damp, plumbing leaks, all the things which all buildings get from time to time. It shouldn't really be used in any application where it is permanently fixed. I am aware that the rest of the world will go complacently using it because it is cheap and because it requires less skilled labour but I won't.
So it doesn't really matter if you prime only, prime and paint, put dpm underneath it, bitumen it, soak it in linseed oil, buy it with a melamine face, whatever - it will still need replacing once a bit of wet has got to it. Plastic or wood.
Tim w
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I'm 100% behind the option of a much better product in that application. On the ridiculous end of the spectrum, how about stainless steel? I know Tim W isn't being silly and he's right. MDF is not the optimum product, but if that is what you're using, paint it all around, sealing as much as possible from the universe and that leaves us with oil-based, piss-coated (thin it 20% or thinner) primer coats that soak in nicely and I wouldn't bother with a primer per se. Use the oil-based paint that you're going to finish with and use *it*, thinned, as a primer. That works for me and has for decades. I'm sure our dear friend Nailshooter has much to contribute and I hope he does.
r
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Tim, your experience is very different than mine. I built two angled floor plates in a box truck. I expected them to be temporary and only used MDF as a pattern and to check concept and dimensions. That was 8 years ago and these unfinished platforms have been exposed to wind, snow, mud, blowing rain, an antifreeze leak, spilled coffee, and heavy foot traffic. So far they haven't shown any of the signs you're talking about.
I was also concerned about using it as unfinished flooring in the shop, so a I nailed down a 4x4 square at the entry door thinking it would not do well and I could eliminate it as a choice. Capping the shop floor is a low priority - that was 4 years ago and the stuff remains unhurt and I would be willing to pull it up and use it as a finished surface.
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Aren't you all really talking about MDO and not MDF?
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On 5/3/2010 12:31 AM, Doug Brown wrote:

This is a case where the OP should get a piece of what he's thinking about using and get it wet and see what happens.
Note that there is a product called "Extira" which is exterior grade MDF, with moisture, rot, and termite resistance and limited swelling. It apparently works pretty well but it costs about the same as Baltic birch. The same company also produces purpose-made trim boards of a similar material under the "miratec" brand in a variety of lengths and widths, which might be cheaper for the intended use and would certainly be easier to handle than full sheets of 3/4" MDF.
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I mistakenly used MDF baseboard in my house. Granted baseboard will take more abuse than your window trim but be advised it chips, dents, flakes easily. Although five years around the kitchen floor, which SWMBO washes at least twice a week, and no signs of water absorption. I used the pre-primed BORG stuff and painted all sides before installing including the miters & copes.
I built raised panel doors using MDF for the panel and poplar for the frame. I painted the MDF with a roller before assembly and ended up repainting them because the paint rolled onto the super-smooth MDF looked too fake. I ended up repainting them with a brush. To me the rolled paint on MDF is too smooth.
Try it both ways and let dry to see which you prefer.
I'm with Robatoy on the spackle. Works much better on MDF than painters putty.
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Limp Arbor wrote:

Kinda depends what you paint it with. latex, enamel. I never use latex on wood or mdf. You can't sand latex
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The Borgs often sell wrapped MDF. Looks primed, but in reality has a condom all over the face of it. I have seen that stuff come off in sheets. Just awful. The primed MDF, a bit better, but still, horrid. Bottom line? Crap is crap no matter how you package it. I made a few dozen kitchen cabinet doors out of panel-routed MDF once. By the time *I* was happy with a finish that *I* was confident in leaving behind at a customer's house, I has spent so much time and money on materials that it was simply outrageous. They are still nice 15+ years later and they damned-well better be, there are my personal sweat and miscellaneous body parts ground into that finish. NEVA again! *spits on ground*
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Picasso wrote:

Whatever you prime/paint it with, make sure it's oil-based. The water-based primers/paints will raise the routed "grain" making it fuzzy and not-smooth.
I learned this the hard way installing a roomful of raised-panel wainscoting constructed of wainscoting.
~Mark.
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Woody wrote:

Make that "wainscoting constructed of MDF."

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