# Finishing MDF



## TheLaw612 (Jan 17, 2012)

I am going to be building a pair of Overnight Sensations thanks to the awesome kit that is available from Erich at DIYSoundGroup. They will be used as computer speakers for music/games. This will be my first adventure into DIY so I am looking for recommendations for a pretty simple way to finish MDF to get a decent black, semi-gloss finish. Since this is my first time, I'm not really looking for a piano black finish - maybe on the next build.

I'm assuming I would sand the MDF with maybe 150 grit, then apply a primer, then sand again, then apply a couple coats of black spray paint, sanding in between? Can anyone recommend a good spray paint finish? Also, would I need to add a clear coat at the end? Thanks a bunch.


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## Anthony (Oct 5, 2006)

I have an hvlp sprayer, so I don't normally shop for spray paint, but I have painted a lot of MDF over the years.

The steps, as I remember them:
1) Sand all the joint edges smooth with 100 grit.
2) Use vinyl spackle or bondo to fill in any gaps, no matter how small.
3) Re sand after it dries.
4) Using a spray primer, make a first pass over all the "ends" of the MDF that are exposed.
5) Prime the rest of the MDF in light passes
6) Re-prime the ends again after doing the flat surfaces.
7) Allow to dry and repeat as necessary. The edges soak up primer and paint by the bucket, so it is important to hit it first and let that set in and dry before continuing. If it looks like it needs a bit more, it probably does. Primer is cheaper than paint and sands easier, so always err on the side of more priming. Several light passes and waiting 15 minutes is better than glopping it on.
8) Lightly sand the primer with 220 grit and wipe down. If you sanded through any primer touch up as necessary.
9) Apply coats of paint like the primer: raw edges first, flat surfaces, then a quick light pass over the edges again.
10) Repeat coats as necessary. For paint usually 2 is enough if you used enough primer. Sand lightly between coats if anything looks rough or uneven, but only 220 grit or so. Wipe clean.
11) Optional: top coat with a water-based satin or semi-gloss polyurethane in the same manner.


It sounds like a lot of steps, but sprayed finishes are thinner and dry much faster and are so much more forgiving if you do multiple light coats. I recommend the top coat as it will add a lot of protection and guarantee the sheen you want. Sometimes MDF soaks in parts of the paint unevenly and you can have one area glossier than another.

Finally, I usually start with the bottom and do it by itself. Let it dry, flip it, and then do the rest. Those plastic painters pyramids are great for things like this, as they let you get started on the next side while the one you just painted is drying. Occasionally they leave a small dimple in the paint, but that's why I do it on the bottom first.

Good luck and let us know how it goes!


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## fschris (Oct 31, 2009)

Before the spray primer... 

I suggest you d0 a 50/50 mix Water and Wood glue ( tight bond etc)

use a foam roller or foam pad.... do this 2 times. It seals in the MDF. cover the whole thing with it

Then do the primer...

Sand the primer etc then finish as suggested above

I actually got a OS that I am using for my center channel right now. It was my first DIY and it looks pretty bad! I did not finish it the right way. But I learned a lot after building my MFW15 clones


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## untuned (Mar 29, 2012)

I second Anthony's method as well. If your end results are not what you want you can always veneer them.


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## Jstslamd (Nov 30, 2010)

I second the 50/50 glue water mix too. At the least smear some wood glue on the end grain with your finger even and smooth. If all else fails. PE sells satin black vinyl and black ash vinyl that is simple to do.


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## fschris (Oct 31, 2009)

the MDF will drink up the 50/50 ... you will save a LOT of paint ! it makes a nice hard shell. i am going to try to spray my zaph build with rustoluem spray enamel.
if you go for a can of something I suggest this with a foam roller... i used flat black for my last project wish i had used the semi gloss

http://rustoleum.com/CBGProduct.asp?pid=130


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## robbo266317 (Sep 22, 2008)

You will benefit from sealing with the PVC glue. 
A lot of woodworkers make templates from mdf and glue the edges so it lasts longer when the router roller is run across it.


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## TheLaw612 (Jan 17, 2012)

Thanks for all the awesome info guys. Looks like I'll have to go buy a small foam roller for sure. Maybe I'll practice a few times on some scrap MDF before acutally finishing the OS's. Hopefully it turns out well so I can get it under my belt before I build some new speakers and sub for my HT.


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## untuned (Mar 29, 2012)

Good luck and have fun!:T


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## fschris (Oct 31, 2009)

i suggest you spray the OS... its pretty small... if you are doing a big sub id suggest a can of the rustoluem oil based enamel.


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## JimH (Nov 11, 2011)

Are you just buying the components and not the kit with the flat-pack cabinets? I'm pretty sure the flat-pack cabinets are birch plywood. Not a big difference, but treating the edges is a little different if you are painting.


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## fschris (Oct 31, 2009)

True the OS is birch ! I built the OS MTM ... that one was MDF.

I suggest you go for the OS MTM!


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## Anthony (Oct 5, 2006)

I use the Zinsser Bullseye shellac primer on MDF and it seals similar to the 50/50 glue mix. It soaks in, but dries fast, so after 2 or 3 coats of primer it is fully sealed and ready for paint. Nice thing about the shellac is that it is evaporative and is ready to recoat in about 15 minutes, so you can prime everything and get to painting in the same day.

End grain birch plywood will be treated similarly. However, if it is not baltic birch, the plies may have some voids that show on the end. Use Bondo or setting type wood filler (epoxy or the "real wood" stuff, not the vinyl spackle) to fill the voids and sand. It won't need as much extra primer on the edges, but they will soak up a bit more than the faces.

Good luck and keep us posted!


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