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?
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.
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.
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.
Keep the whole world singing . . . .
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.
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
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.
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*
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
I learned this the hard way installing a roomful of raised-panel
wainscoting constructed of wainscoting.
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