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  • finishing

    ok, so since i have an abundance of scrap mdf sitting in the garage, i want to practice my finishing skills. none of the pieces are big enough to do anything super productive with, but a few pieces that are big enough to make some super small cubes or even just some simple right angles. my question is, what do you guys recommend for priming/prepping mdf for painting. i know that the exposed edges of mdf are a pain and act like a sponge to anything wet. any ideas? i want to have a little fun with some scrap stuff before i put all this time and effort into a real project and then put on a terrible finish. thanks!

  • #2
    Re: finishing

    I haven't actually done it myself, but I'm sure that I read here that a 1:1 solution of wood glue and water is one of the preferred methods of sealing the end grain of mdf prior to painting. Apply with a brush, let it dry, sand it. Prime using a high build primer, etc.

    I believe that one of Wolf's blogs has his method described in some detail, and he gets some pretty amazing finishes. You might want to check there.

    Mark
    You go your way, I'll go mine. I don't care if we get there on time.

    ~Pink Floyd

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    • #3
      Re: finishing

      Originally posted by Mark65 View Post
      I haven't actually done it myself, but I'm sure that I read here that a 1:1 solution of wood glue and water is one of the preferred methods of sealing the end grain of mdf prior to painting. Apply with a brush, let it dry, sand it.
      I have done it many times. Works great. Usually, I put 2-3 coats on the end grain and then one over the ENTIRE surface. The glue mix does adhere a little bit even to the face, so the once-over helps reduce visible imperfections when spraying gloss paints.

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      • #4
        Re: finishing

        I would consider using straight glue on the edge grain and a 50/50 mix over the rest of the surfaces.

        I tried a mix on the edges once and wasn't too happy with the results.

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        • #5
          Re: finishing

          I tend towards the thin side. Maybe 1/3 glue. Probably also why I need to use 3 coats, but it does a good job of hardening the fibers.

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          • #6
            Re: finishing

            Originally posted by Dirk View Post
            I tend towards the thin side. Maybe 1/3 glue. Probably also why I need to use 3 coats, but it does a good job of hardening the fibers.
            I've always used straight Elmer's white glue in the edges of MDF, and nothing less. I have never had any trouble with edge-grain since I've started doing it this way. Once it dries, I hit it with 150/220/400 grits and it's smooth as necessary for primer. I could understand if you used Elmer's wood-glue why you would want to dilute it. With the white glue there is no need.
            Later,
            Wolf
            "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
            "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
            "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
            "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

            *InDIYana event website*

            Photobucket pages:
            https://app.photobucket.com/u/wolf_teeth_speaker

            My blog/writeups/thoughts here at PE:
            http://techtalk.parts-express.com/blog.php?u=4102

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            • #7
              Re: finishing

              Great suggestions thus far. What wolf said about white elmers vs wood glue is good advice. Diluting normal white elmers will require a few more coats.

              I use elmers wood glue, 75% or 100% on the edges and roundover/routered areas, and a 50/50 everywhere else.

              On the topic of primer, what are you guys using? I would suggest almost always going with an oil based primer, as all paint will adhere OK to it, while some paint goes on very poorly over a latex primer. That being said, a thick latex primer is great for hiding sanding marks and imperfections, but it usually requires a little more sanding before applying paint unless you sprayed it on or used nice brushes (And I for one use the $0.99 pieces from Home depot).

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              • #8
                Re: finishing

                Originally posted by jdclark View Post
                On the topic of primer, what are you guys using? I would suggest almost always going with an oil based primer, as all paint will adhere OK to it, while some paint goes on very poorly over a latex primer. That being said, a thick latex primer is great for hiding sanding marks and imperfections, but it usually requires a little more sanding before applying paint unless you sprayed it on or used nice brushes (And I for one use the $0.99 pieces from Home depot).
                I use automotive heavy or high build primer, sprayed on. I don't know if it is oil-based or latex, but it sands easily.

                To get a smooth finish, you need many cans and coats, with sanding in between: bring elbow grease and patience.

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                • #9
                  Re: finishing

                  Originally posted by jclin4 View Post
                  I use automotive heavy or high build primer, sprayed on. I don't know if it is oil-based or latex, but it sands easily.

                  To get a smooth finish, you need many cans and coats, with sanding in between: bring elbow grease and patience.
                  probably oil based. All the auto paints I know of are.

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                  • #10
                    Re: finishing

                    Does the coat of wood glue help to increase the MDF's resistance to dings and chipping at all?

                    Louis

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                    • #11
                      Re: finishing

                      Sanding sealer also works well on MDF.

                      The last time this discussion came up some objected to diluted wood glue due to the high water content. Another suggest was shellac. That is what I will try on my next box.

                      Comment


                      • #12
                        Re: finishing

                        Originally posted by jdclark View Post
                        Great suggestions thus far. What wolf said about white elmers vs wood glue is good advice. Diluting normal white elmers will require a few more coats.

                        I use elmers wood glue, 75% or 100% on the edges and roundover/routered areas, and a 50/50 everywhere else.

                        On the topic of primer, what are you guys using? I would suggest almost always going with an oil based primer, as all paint will adhere OK to it, while some paint goes on very poorly over a latex primer. That being said, a thick latex primer is great for hiding sanding marks and imperfections, but it usually requires a little more sanding before applying paint unless you sprayed it on or used nice brushes (And I for one use the $0.99 pieces from Home depot).

                        I typically use the automotive variety. I go to Advanced Auto or AutoZone and get the grey Duplicolor "High Build Primer". Your prep is the most important. I sand the glued edges with 150/220/400 grits once it's dry, and the wntire surface is usually lightly sanded with 220 or 250 grit. You always sand before applying paint. Period. I usually work with 150 to take off the rough raised-grain nodules that appear, and then 250 and 400 grits. It should be dry before you sand, and it should be very smooth before paint.

                        If you want to see how I do most of mine with spray-paint, I have the writeup in my Blog linked below.
                        Later,
                        Wolf
                        "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                        "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                        "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                        "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                        *InDIYana event website*

                        Photobucket pages:
                        https://app.photobucket.com/u/wolf_teeth_speaker

                        My blog/writeups/thoughts here at PE:
                        http://techtalk.parts-express.com/blog.php?u=4102

                        Comment


                        • #13
                          Re: finishing

                          Originally posted by Ludo View Post
                          Does the coat of wood glue help to increase the MDF's resistance to dings and chipping at all?

                          Louis
                          It does do that a bit. I've had dings in a couple cabinets, and only the paint chipped, no ding in the actual MDF body, and the primer was still intact underneath. Now if you drop them down the stairs........
                          Later,
                          Wolf
                          "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                          "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                          "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                          "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                          *InDIYana event website*

                          Photobucket pages:
                          https://app.photobucket.com/u/wolf_teeth_speaker

                          My blog/writeups/thoughts here at PE:
                          http://techtalk.parts-express.com/blog.php?u=4102

                          Comment


                          • #14
                            Re: finishing

                            Originally posted by eyekode View Post
                            Sanding sealer also works well on MDF.

                            The last time this discussion came up some objected to diluted wood glue due to the high water content. Another suggest was shellac. That is what I will try on my next box.
                            I use de-waxed shellac (Zinsser Bull's Eye Seal Coat sanding sealer) to seal the MDF. The lack of wax improves adhesion of paint.



                            The objection to using water was that the MDF would swell with too much water and lose strength where it was allowed to penetrate. I've also found that sanding glue isn't as easy as sanding the de-waxed shellac.

                            Both methods work though - obviously.

                            Here's one recent observation...even de-waxed shellac will swell MDF somewhat if you "feed" too much of it into the edges. MDF wants to "drink" the thin, alcohol-based shellac into edges like a sponge. After drying, joints that were sanded flush were slightly raised and needed sanding again.

                            In the past I let it "drink in" the sealer until it wouldn't take more (and that's a LOT). In now believe that's the wrong way to do it. Next time, I'm going to try several lighter-coats with drying in between to seal MDF so sealer won't penetrate so far and cause the slight swelling. This likely holds true with diluted glue treatments too.
                            Bill Schneider
                            -+-+-+-+-
                            www.afterness.com/audio

                            Comment


                            • #15
                              Re: finishing

                              you guys are great! this is why i asked. i have those scrap pieces laying around, so i can try a few different methods and give em a whirl to see what pans out the best for me. thanks so much to everyone for thier input and help!

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