Ugly MDF Finish

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  • Ugly MDF Finish

    I recently completed a mobile base for the BT3100 and also built some matching cabinets for the shop. They are 3/4 inch MDF with poplar trim on the edges and birch plywood drawer and door faces.

    All went well until I coated the MDF with an oil-based polyurethane clear satin coating, which was just brushed on. Instead of getting that nice rich brown color (like Rod's) I ended up with a mottled finish (see pictures). One shows the top of a cabinet and the other shows the inside of a drawer built with MDF. Since all MDF surfaces will be out of sight, under something or the inside of a drawer, I am not too concerned about appearances at this time. And, admittedly, I was not overly careful about applying the finish to these surfaces -- I won't say smeared on, but. . .

    However, in the future I may want to have MDF surfaces visible and need advice on proper finishing of the MDF so as to avoid the mottled appearance. I am also a little concerned that the finish may not adhere to the MDF as well as it should.

    As always, suggestions and comments are most welcome.

    Thanks, Steve

    PS: I know those photos are making Rod and others cringe so I apologize in advance.
    Last edited by steve-norrell; 11-30-2008, 07:03 PM.

  • #2
    steve,

    i am not sure where you went wrong or what you did wrong. however i can tell you how i finished my mdf on my work bench (pictures were recently posted in finished projects section).

    basically, when i was done cutting the mdf for the talbe top, sides or the drawers and doors, they all had some marks on them left behind by the the gray paint on the bt3 top and a few black marks here and there from hammer dragging on it or something. i sanded all the surfaces down to 220 using the random orbital sander. then with a air hose and my hand cleaned the surface and then took a damp towel and rubbed them over the entire surfaces and you wont believe how much more find dust came off with that.

    this is where i tried three different methods:
    • everything but the doors, drawers and top: i thinned oil base poly 50-50 and then sprayed it on with an hlvp. i ended up doing 3 or 4 coats because the mdf was just sucking everything up. the fninsh is not bad. after the first spray i changed the mixture to more like 60-40 and even tried 75-25 (poly-thinner)
    • for the drawers, i sealed the mdf with min-wax sealer, let it dry and sanded it. then i sprayed the mdf with 60-40 mix two coats in total (but i sprayed the heck out of them because everything was getting sucked in during the first coat). the finish still wasnt like rod's.
    • for the doors and top. i sealed them twice. i sanded each time after it dried up. this time i used min wax wipe on poly. after the first coat, there were areas where the mdf still sucked it in. then during the second coat, i was a little more liberal in the amount i put on and then the third coad, i put on even more, almost like a very wet look. in between the coats, i wet sanded using 500 grit paper.
    it was like 100+ degree when i was applying these finishes. i had no choice, the wheather here is pretty hot during the summer. for what people on this site have said, they mostly recommended poly for work stuff because the finish is more resistant (from what i gather).

    for indoor furniture (even some desk top organizer made out of mdf), i plan on using a couple of products that stan and rags suggested that they have had great results with:
    • Sherwood Moisture Resistant Lacquer
    • resisthane
    sorry for going on and on, i hope this will help.

    good luck!
    _________________________
    omar

    Comment


    • #3
      Awaiting Rod to chime in, but I am guessing Sealer.
      She couldn't tell the difference between the escape pod, and the bathroom. We had to go back for her.........................Twice.

      Comment


      • #4
        I suspect that the sealer is the answer. Question would be what to use as a thinner. Ordinary "paint thinner" would probably not work.

        footprintsinconc: I did notice your posts and ogled enviously. Indeed, while I had been thinking about posting this question, your pictures prompted me to do so. I don't have spray facilities and I must work in an attached garage, so spraying is probably out of the question anyway. But, I can't think of any reason why a diluted finish couldn't be brushed, rolled, or even wiped on.

        LinuxRandal: I would be willing to bet my social security check (or a small part of it) that you are right. I am assuming that diluted polyurethane can act as a sealer.

        Do you think I should delete the pictures before Rod sees them?

        Regards, Steve.
        Last edited by steve-norrell; 09-08-2007, 06:16 PM. Reason: Added note.

        Comment


        • #5
          Sorry if this is too simple

          I use WATER based poly ONLY :
          • Single coat, unsanded for a "little grip" (miter fence faces)
          • Multiple coats, fine sanded in between, then waxed - for sliding surfaces.
          BTW I only ever tried oil based once - got the same result you show
          Downunder ... 1" = 25.4mm

          Comment


          • #6
            Originally posted by RodKirby View Post
            Sorry if this is too simple

            I use WATER based poly ONLY :
            • Single coat, unsanded for a "little grip" (miter fence faces)
            • Multiple coats, fine sanded in between, then waxed - for sliding surfaces.
            BTW I only ever tried oil based once - got the same result you show

            Simple is better. Will give it a try.

            Thanks, Steve

            Comment


            • #7
              Originally posted by RodKirby View Post
              I use WATER based poly ONLY
              The solvents in oil-based poly probably react to some extent with the resins, solvents, or other chemicals in the MDF, causing either a strange cross-linking in the poly or a slight meltdown in the surface of the MDF, or both.

              Something similar happened recently when I used some advanced 'sticky remover' to take off the residue of a label on a plastic putty knife. It wasn't coming off, so I applied a little more, and finally I realized that the combination was actively MAKING something that looked like label residue.

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              • #8
                Like Rod I only did one cabinet with oil. Most definately water based poly.
                Scott
                "The Laminate Flooring Benchtop Guy"

                Edmonds WA

                No coffee, no worky!

                Comment


                • #9
                  Originally posted by steve-norrell View Post
                  footprintsinconc: I did notice your posts and ogled enviously. Indeed, while I had been thinking about posting this question, your pictures prompted me to do so. I don't have spray facilities and I must work in an attached garage, so spraying is probably out of the question anyway. But, I can't think of any reason why a diluted finish couldn't be brushed, rolled, or even wiped on.
                  with an hvlp gun you shouldnt be getting much overspray. i also did it in attached garage. it could be brushed on aswell, i just didnt want to spend all that time. i used the thinner that is used to dilute oil based finishes.

                  regards,
                  _________________________
                  omar

                  Comment


                  • #10
                    I think that one should go in the articles, or Lorings, faq's.

                    I am sorry so many of us will have to relearn this, from not knowing.


                    Thanks from one person who has been spared.
                    She couldn't tell the difference between the escape pod, and the bathroom. We had to go back for her.........................Twice.

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