[PDR]

Yep, thats the stuff.

Seams, joints, pin nail holes, screw holes and scratches.

[Peter J]

Here's something to consider;

Wood glue has water in it, adding water to glue makes it more watery. Water raises grain and swells the particles in MDF. I suppose after a few coats of glue and water mixture you could get some sort of a "seal", but glue is not really intended to be or designed for sandability or to be used as a filler, so it creates another problem.

 I know this is commonly done, but it seems counter-intuitive to use water bearing anything if one is trying to smooth something out that's made of wood products.

Even though it's designed for autobody work, polyester fillers and paints have the qualities one is looking for in wood finishing. As to primer, high build catalyzed polyester will give great results, but failing that an air dry sandable (meaning it powders when sanded) primer is your best bet, that will almost always mean solvent or oil base.

And in the FWIW department, butt seams will almost always show at some point to some degree...even under veneer! It's just the way wood products respond to atmospheric conditions. Really depends on what level of finish you're after.  If you want it to be dead level and  super glossy and seamless looking like a Wilson or some such, the prep is extensive and the material to get there spendy.

[django11]

And in the FWIW department, butt seams will almost always show at some point to some degree...even under veneer! It's just the way wood products respond to atmospheric conditions. Really depends on what level of finish you're after.  If you want it to be dead level and  super glossy and seamless looking like a Wilson or some such, the prep is extensive and the material to get there spendy.

+1 on that.  Keep in mind the humidity levels when you are doing the work.  For example, I did some finish work on my staircase when I moved in to my place during the most humid time of the year.  A butt joint in mdf sanded smooth and painted stands proud by almost 1mm in the middle of heating season when everything has dried out...

[SteveRB]

drywall filler and a very large very flat sanding block works for me. then priming and painting.

[MLS]

Thanks guys.  Learned something new today.

Scott

[bdp24]

I found a product on McFeely's website.....wood filler by Famowood. Made from sawdust (I used to have a pro furniture maker/woodworker friend who made his own that way. When he went pro as a musician he turned his woodshop into a recording studio, damnit), in ten different colors. I ordered Natural to match the MDF. $5.85/pint, a buck to ship.

[dBe]

I avoid the whole problem by using Medex, an exterior rated version of MDF.  It is twice the cost, but worth it.  I put a 3/4" x 3" x 6" in the back of my truck and left it for a year just to see how durable it really is.  The only change was that it got dirty.  No swelling, no dimensional changes.  Butt joints with Titebond II are a breeze.  All you have to do is sand the joint flat with 120 grit and it doesn't telegraph the joint through paint due to glue creep.  It is also a lot stiffer than MDF and a little denser.  Good stuff.

Dave

[bdp24]

Thanks Dave, I'll look for it.

Wow. How come everybody isn't using Medex? This stuff looks really good, with greater density, stiffness and strength, screw pull-out capabilities, etc., than bog-standard MDF. On their site is a great tutorial on the specifics of surface prep and painting of MDF, including the types of primer, base coat, and top coat best used on MDF. They even offer a version of the stuff covered on one or both sides with a plastic laminate (available in white, black, maple, and cherry) for those who don't want to deal with painting, staining, or laminating. I'm gettin' me some!

[devinkato]

Here's something to consider;

Wood glue has water in it, adding water to glue makes it more watery. Water raises grain and swells the particles in MDF. I suppose after a few coats of glue and water mixture you could get some sort of a "seal", but glue is not really intended to be or designed for sandability or to be used as a filler, so it creates another problem.

I agree.  The water/glue mixture has never worked well for me.  The absolute best for me is alcohol based shellac primer.  It sinks in well to MDF grain, and then evaporates very quickly, leaving a nice hard sealed surface that is really easily sandable.  I really encourage anyone who is looking to get a nice finish on MDF to try it out.  I use it after I've sanded all edges flat, and done any router work (roundovers, chamfers, etc).  Works great every time.  I usually follow up this sealing primer coat with another go of sanding, then hit the cabinet with a nice coat of filler primer.  If I did a good job of sanding before and after the sealing primer, I won't have to sand the filler primer at all, and I can move straight onto shooting the finish coat..

[Captainhemo]

I agree.  The water/glue mixture has never worked well for me.  The absolute best for me is alcohol based shellac primer.  It sinks in well to MDF grain, and then evaporates very quickly, leaving a nice hard sealed surface that is really easily sandable.  I really encourage anyone who is looking to get a nice finish on MDF to try it out.  I use it after I've sanded all edges flat, and done any router work (roundovers, chamfers, etc).  Works great every time.  I usually follow up this sealing primer coat with another go of sanding, then hit the cabinet with a nice coat of filler primer.  If I did a good job of sanding before and after the sealing primer, I won't have to sand the filler primer at all, and I can move straight onto shooting the finish coat..

I am going to try this wehen I build my  OB7's shrrtly .  .  Thanks for the tip

-jay

[bdp24]

Likewise!

[Big Red Machine]

superglue

[bdp24]

About sanding.....are ya'll using electric sanders, or doing it by hand? I got spoiled using a DeWalt ROS when I was putting in some hours working for that friend, making things like dining room tables (no veneer over MDF, solid hardwoods).

[stevenkelby]

I do it by hand with this awesome 1/4 sheet  sanding block:

http://www.amazon.com/3M-Sanding-Block-2-75-Inch-5-25-Inch/dp/B00002N8T8

[Peter J]

Air power is my fave. The key is in the pad.

http://www.hutchinsmfg.com/View-Product.aspx?group_id=418693

http://www.dynabrade.com/dyn10/content.php?page=myind#woodworking

[LoganTubester]

Bondo ..... fills in the gaps, sands down super smooth, and takes primer or contact cement great.  Easy to see where you apply it.  Not really a choice if you want to use a clear coat on the MDF .... but great for solid finishes or veneers.

[bdp24]

Air power is my fave. The key is in the pad.

http://www.hutchinsmfg.com/View-Product.aspx?group_id=418693

http://www.dynabrade.com/dyn10/content.php?page=myind#woodworking

Damn PJ, those are nice. I'm not worthy!