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  • Bondo/elmers/gluewater on MDF...

    Hello all:

    I know the question of how to prep and paint MDF has been covered and covered rather exhaustively. However, I haven't seen a ton of good pictures on prepping the MDF for painting.

    I'm painting my RBs right now. I do a coat or so after work and it's taking forever. This is how I did it...

    First I used wood filler on all the joints and countersunk screws then sanded it down. Then, I did 3 coats of spray on primer and sanded after2 and 3. Then I sprayed 4 layers of paint, sanding between 2 and 3 and after 4. I'll soon coat it with glossy clearcoat.

    The result... it... well... it sucks. I've not done this before and it turned out horrible for the cabinets. However, the baffle looks great. I used a textured paint and satin clearcoat and it's money. But, the cabinets look like junk. The end grain is bumpy and you can see the joints. So, I don't think it's my painting that sucks (because the baffles are great) but it's clearly my prep on multiple pieces joined together.

    Could people please post any pictures they have of their technique for sealing/coating their boxes for us newbies that are visual learners?

    Thanks in advance for any/all replies.

    Oh, and congrats on making through and unnecessarily long post.

    Below are images of my progress (almost finished cabinets are not show to spare me the shame):






  • #2
    Re: Bondo/elmers/gluewater on MDF...

    The end grain of the MDF has to be sealed prior to any primer/paint being applied. I use Wolf's method of rubbing in Elmer's white glue with my fingertip on all exposed end grain. It dries pretty quickly, so I usually do this 4 times before sanding. I also overlap the seam with the Elmer's. I hand sand with 120 - 220 - 320. Then I primer. If you do it correctly, the end grain will have a hard, shiny surface like the uncut portion. Check out Wolf's blog on painting MDF - he has done some remarkable finishes.
    Audiophiles listen to the equipment, not the music.

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    • #3
      Re: Bondo/elmers/gluewater on MDF...

      Problem is that you used latex based primer. Water and MDF do not mix. You MUST use oil based primer to avoid bumpy finish on MDF. You might get lucky sometimes and not have the problem, but do you really want to take the chance?

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      • #4
        Re: Bondo/elmers/gluewater on MDF...

        To be more clear, you MUST use something that's not water soluble. Glue/water wash works because it dries hard. I use shellac because it's alcohol based and dries extremely fast. As to seams, a little spackling works as well as wood filler or bondo; it's all in what sandable filler you have available.

        I'm curious, though, what you did differently on the baffles, since they are end grain on all the roundovers, yet came out nice? In many applications, that's all anyone sees, regardless.

        Sorry, no pics of my RB's.

        HAve fun,
        frank

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        • #5
          Re: Bondo/elmers/gluewater on MDF...

          Stay away from water base paints....MDF just absorbs this stuff and makes it swell up and things just get **** ugly from there on out?

          MDF has to be sealed before painting! I ususally seal the end grain first! Some just rub on straight wood glue, others dilute wood glue. I usually dilute Titebond 2(sets fast) down to the consistancy somewhere between the consistancy of milk & cream! Just brush it on the end grain, let it set up and dry , lightly sand, then apply a second coat on end grain and the entire cabinet. Once dry, I give it a light sanding, then apply clear poly, about 4 coats with dry and light sanding between coats. More coats maybe needed depending on how smooth a surface you want. Min Wax Seal & Sand can also be used instead of Poly also! Make sure all cracks/crevices etc. are filled with bondo, wood filler or whatever your using and sanded smooth before any sealing!
          Takes time, but the end results are allways nice

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          • #6
            Re: Bondo/elmers/gluewater on MDF...

            my joints didn't show up until after the paint as well. primer didn't show them and you could not feel them. most people won't see them unless you tell them to look for them though.

            i believe it has to do with MDF and we might get better results with plywood. i don't know but i'm living with it. you might have to resand smooth and paint again.:(
            david golemba
            chesterfield michigan

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            • #7
              Re: Bondo/elmers/gluewater on MDF...

              I hope you have a random orbital sander or you will cry by the time you are done sanding.

              Prep process
              1st.. rub bondo into all seams
              2nd wait for 1st bondo to try and then repeat
              3rd rub undilluted elmers wood glue over entire mdf surface. Rub 2nd coat into end grain
              4th sand through grits to 220 (80,120,220)
              5th 3-4 coats of sandable primer allowing each coat to dry
              6th sand through grits to 400. (80,120,220,400)
              7th On to paint process

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              • #8
                Re: Bondo/elmers/gluewater on MDF...

                FBOV:

                I didn't do a thing different on the baffles. I think the difference came in the fact that I'm using a textured paint on the baffle plus there's a roundover into the endgrain instead of a **** joint. The "texture" added by the painting of the endgrain just matched the textured paint.

                EVERYONE:

                Thanks for all the responses. Amazing.

                I seems that the painting process is the longest of all the steps in the build process. In my spare moments in the evening it would take me a few weeks to seal/prime/paint/coat. I have to do one at a time due to space constraints on my small apartment balcony. Not to mention my "paint box" doesn't look too good out there.

                Again, thanks for all the info! The next round will be better. And the good news is that I can always make new cabs and take my sweet time and just transfer the drivers, crossovers and binding posts to new cabs later.

                Comment


                • #9
                  Re: Bondo/elmers/gluewater on MDF...

                  Originally posted by MidKnight View Post
                  FBOV:

                  I didn't do a thing different on the baffles. I think the difference came in the fact that I'm using a textured paint on the baffle plus there's a roundover into the endgrain instead of a **** joint. The "texture" added by the painting of the endgrain just matched the textured paint.

                  EVERYONE:

                  Thanks for all the responses. Amazing.

                  I seems that the painting process is the longest of all the steps in the build process. In my spare moments in the evening it would take me a few weeks to seal/prime/paint/coat. I have to do one at a time due to space constraints on my small apartment balcony. Not to mention my "paint box" doesn't look too good out there.

                  Again, thanks for all the info! The next round will be better. And the good news is that I can always make new cabs and take my sweet time and just transfer the drivers, crossovers and binding posts to new cabs later.

                  Judging by the pic's, your baffles do look nice...great work! Good things come to those who have patience!

                  Comment


                  • #10
                    Re: Bondo/elmers/gluewater on MDF...

                    Here is a thread over on DIY Audio addressing this very subject.

                    http://www.diyaudio.com/forums/showt...652&highlight=

                    Regards,

                    Dennis

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                    • #11
                      Re: Bondo/elmers/gluewater on MDF...

                      my joints didn't show up until after the paint as well. primer didn't show them and you could not feel them. most people won't see them unless you tell them to look for them though.
                      You may have rushed the process. Your fillers may not have been completely dry. As they dry they will shrink. Same goes for primer. Especially if you lay it on thick. The cracks are the deepest fill so they may have shrunk the most.
                      Dave
                      http://www.pellegreneacoustics.com/

                      Trench Seam Method for MDF
                      https://picasaweb.google.com/101632266659473725850

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                      • #12
                        Re: Bondo/elmers/gluewater on MDF...

                        the sandable primer drys very fast. remember, glossy paint will always show off the smallest of flaws. i used a primer-sealer and then spray paint and then i used water based spray for the final finish its really great.


                        ps did you forget to recess your tweeter?

                        your baffle looks real nice. you can handle your router real well.



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                        Quote Originally Posted by hongrn. Oct 2014
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                        • #13
                          Re: Bondo/elmers/gluewater on MDF...

                          I got one word for you

                          Veneer. I find it so much asier thn paint. ALso, if i plan on painting something, i veneer it with scraps now just to seal the grain
                          Check out my website: www.uberstealthaudio.com.
                          Now offering cnc cut baffles and other speaker cabinet parts

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                          • #14
                            Re: Bondo/elmers/gluewater on MDF...

                            Sherwin Williams "Icing" p/n US2606 pourable brushable polyester autobody filler putty is good for edge sealing and light filling.

                            "Our Nation’s interests are best served by fostering a peaceful global system comprised
                            of interdependent networks of trade, finance, information, law, people and governance."
                            - from the October 2007 U.S. Naval capstone doctrine
                            A Cooperative Strategy for 21st Century Seapower
                            (a lofty notion since removed in the March 2015 revision)

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                            • #15
                              Re: Bondo/elmers/gluewater on MDF...

                              I'll second the Shellac... I think any water you get into the MDF will take a while getting back out. The Alcohol that is Shellac's base evaporates much faster, and doesn't seem to cause as much swelling while it's in there.

                              Get a quart (or gallon!) of Zinnser's Seal Coat... and some more denatured Alcohol. Mix the Seal Coat and Alcohol 50/50 in a jar, for a thin, penetrating mix.

                              Brush it on till it stops soaking in. Inside and out. Special attention to the edges. You'll be amazed how much will get drunk up.

                              Wait a few hours to overnight, do it again. Maybe a third time, or 4th. Maybe use full strength for the third coat or 4th... when it slows down absorbing more you know you've done enough. Let it dry at least a day or two.

                              You now should have a well stiffened and sealed layer in the MDF surface. Sand and enjoy how hard it is and how well it sands to flat smooth edges. Coat with glue and veneer, or start laying down some primer and paint layers...

                              Sealing inside and out will help the box to not soak up moisture/lose moisture as the seasons go by, and not have the joints move and show thru the finish.

                              Might be an idea to build the box, and put it somewhere warm and dry for a week or so... before starting the sealing and finishing. There was a guy on here (Aaronpro?) who did amazing sculpted boxes with auto paint finishes.. he finally resorted to a DIY heater box with a couple of 500W light bulbs and a fan as a hot air source, to bake out his boxes for a few days to insure MDF stability...

                              Stabilizing and sealing well should go a long way to keeping the MDF from moving.

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