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  • First time Fiberglass questions

    Well this is my first time using fiberglass and building a sub box. I decided to build the box out of MDF instead of using a fiberglass mold. Didn't want to mess up my car by accident. Now that it is build out of MDF, I want to cover it in fiberglass to make it smooth and rounded so I can paint it.

    My question is:
    1. Is there anything I need to spray or do to the MDF to make the resin stick better to it?
    2. How much hardener per ounce of resin should I use? I don't want to go do high because it is about 90-100 degress F here and I am new to fiberglass.
    3. How many layers should I use to cover the box?


    Thanks
    Ill post some pics up when I am done.

  • #2
    im new to the whole fiberglass scene myself, but heres what ive gathered through my studies:

    1) i know the resin likes to stick to fleece, so maybe that? thats what people use if they are just building a box from fiberglass at least.

    2) cant really help you there.. sounds like googling territory to me.

    3) ive read that when making a fiberglass enclosure, you put enough layers on it to where you can stand on it without it flexing. dont know how to measure that if youve already build a box from wood tho. -shouldnt need too many layers since the MDF is pretty strong stuff.


    >>on a side note, why dont you just get some sandpaper/electric sander and round off the edges and paint it? that would be MUCH MUCH easier and would probably look better in the end. if you wanna fill in any gaps (i.e. inbetween boards and what not) , i recommend epoxy putty by Loctite (found at lowes). use that to fill in gaps and then you can sand it down. sorry im not an expert, just figured my answer is beetter than no answer

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    • #3
      Thanks for your answers.

      As for your side note, I want to start getting into fiberglass. So I can make some more **** for my car and other side projects. This is basically a learning experience. Thanks though.

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      • #4
        It should stick fine to the MDF. The stuff I used said 14 drops per ounce. It was around 70 here and I used around 8-10 drops to give it a bit more working time.

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        • #5
          Originally posted by TimmyM View Post
          It should stick fine to the MDF. The stuff I used said 14 drops per ounce. It was around 70 here and I used around 8-10 drops to give it a bit more working time.
          Allright. Thanks

          Anyone know about how many layers I should do?

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          • #6
            if its purely cosmetic, and you want it all smooth and shiny, you should look into mixing body filler with the fiberglass resin 5:1 ratio. check out this... http://www.the12volt.com/installbay/...TID~74519~PN~1 it would give you a nice smooth surface to paint onto the box.

            now, if you are going to do more shaping to the box first, youll need to throw down a few layers first. test by pressing it with your thumb. its your judgement on how thick it needs to be if its just a cosmetic piece

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            • #7
              Originally posted by roflcopter View Post
              if its purely cosmetic, and you want it all smooth and shiny, you should look into mixing body filler with the fiberglass resin 5:1 ratio. check out this... http://www.the12volt.com/installbay/...TID~74519~PN~1 it would give you a nice smooth surface to paint onto the box.

              now, if you are going to do more shaping to the box first, youll need to throw down a few layers first. test by pressing it with your thumb. its your judgement on how thick it needs to be if its just a cosmetic piece


              Thanks a lot. I might try the body filler. Not sure though.

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              • #8
                one question that may help:

                is the box in it's current state sealed completely?

                So, if you are just putting the fiberglass over the box to get a really smooth surface, but it's only cosmetic, you would be fine with one coat. If there are gaps (larger than 1/2") then the fiberglass needs to be between 4 and 7 layers depending on the weight of the mat. Post a pic of the box, so we can see what you've got so far. That will help us help you a lot better.
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                • #9
                  Originally posted by monkeyracer View Post
                  one question that may help:

                  is the box in it's current state sealed completely?

                  So, if you are just putting the fiberglass over the box to get a really smooth surface, but it's only cosmetic, you would be fine with one coat. If there are gaps (larger than 1/2") then the fiberglass needs to be between 4 and 7 layers depending on the weight of the mat. Post a pic of the box, so we can see what you've got so far. That will help us help you a lot better.
                  I can't say it is sealed 100% because I cannot fit the silicon caulk under to get the top part of the box. Thats one of the reasons I am putting the fiberglass over as well as for cosmetic. The biggest gap is less than 1 mm. Which I think would be easily covered by the fiberglass in one of 2 coats (thats what I think).

                  Here are some pics:






                  Also, when I add more layers, do I need to keep putting more fiberglass cloth/mat over for each new layer?

                  Thanks

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                  • #10
                    you could seal the joints in the box with some fiberglass resin. just use a paintbrush and brush it into the joints through the sub hole.

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                    • #11
                      The mat and fiber give it strength. If you're not using the fiberglass for structural purposes then you don't need the mat each time. But are you just planning to paint resin to the outside of this box and smooth it down? Are you going for curves? Ever thought about simply carpeting the boxes?

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                      • #12
                        I honestly think that fiberglassing the box you have is really more work than it will be worth in the end.

                        I know you want to learn more about using fiberglass, and want to seal the box, but I really think that the result you are after is not easy, especially for a newbie.

                        To seal the box, you should build the box from the sub hole piece first, that way you can seal the opposite side from the sub hole. Silicone, wood glue, whatever seals the air would be sufficient. For the outside, if you want a perfectly smooth surface to apply vinyl or carpet, or even paint, you can countersink the screws, and then use bondo or some kind of filler, and sand it down.

                        Fiberglass is usually better used when you want more curvy shapes, and in fact it's strongest when it's curved.
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                        • #13
                          Originally posted by guizai View Post
                          The mat and fiber give it strength. If you're not using the fiberglass for structural purposes then you don't need the mat each time. But are you just planning to paint resin to the outside of this box and smooth it down? Are you going for curves? Ever thought about simply carpeting the boxes?
                          Yes I do plan to paint it after I fiberglass it/sand it down. I don't really like the look of carpet or I would use it.


                          Originally posted by monkeyracer View Post
                          I honestly think that fiberglassing the box you have is really more work than it will be worth in the end.

                          I know you want to learn more about using fiberglass, and want to seal the box, but I really think that the result you are after is not easy, especially for a newbie.

                          To seal the box, you should build the box from the sub hole piece first, that way you can seal the opposite side from the sub hole. Silicone, wood glue, whatever seals the air would be sufficient. For the outside, if you want a perfectly smooth surface to apply vinyl or carpet, or even paint, you can countersink the screws, and then use bondo or some kind of filler, and sand it down.

                          Fiberglass is usually better used when you want more curvy shapes, and in fact it's strongest when it's curved.
                          so your just recommending to use bondo to give it a smoother look and then paint it? How would I seal it from the inside though? Should I use fiberglass resin and brush it on in the inside? Like roflcopter said?

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                          • #14
                            Originally posted by PBFreak817 View Post
                            so your just recommending to use bondo to give it a smoother look and then paint it? How would I seal it from the inside though? Should I use fiberglass resin and brush it on in the inside? Like roflcopter said?
                            I would take the panel opposite the sub hole off, then apply silicone sealer around all the edges. Then, put that panel back on, and apply the silicone to the remaining edges, since you will have access to them from the sub hole. Once it's sealed, I would make sure no screw head protrudes past the mdf, and fill all dips and crevices with bondo, then sand down to a smooth finish. Next, I would use a high-build primer, then sand down, and start painting the box. The smoothest finishes require wetsanding, but you'll want to make sure that the box is sealed and that the bare MDF doesn't get wet. If it does, it will swell and ruin the entire project.

                            Adding the FG to the box really only complicates things. You would need to make sure there aren't any air bubbles, and once it's dry and cured, you would still need to use filler to get it perfectly smooth.

                            One thing I noticed with your design is you used braces on the corners. On a box this small, that is really not necessary. It appears you've got 1/2" mdf (recommend 3/4", but 1/2" will suffice, but nothing thinner really) and there is enough on the edges to pre-drill and just screw into the edges of the wood. So, I would remove all the little corner braces you have, and that would make sealing it a whole lot easier.

                            Hope this helps a little bit.
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                            • #15
                              Originally posted by monkeyracer View Post
                              I would take the panel opposite the sub hole off, then apply silicone sealer around all the edges. Then, put that panel back on, and apply the silicone to the remaining edges, since you will have access to them from the sub hole. Once it's sealed, I would make sure no screw head protrudes past the mdf, and fill all dips and crevices with bondo, then sand down to a smooth finish. Next, I would use a high-build primer, then sand down, and start painting the box. The smoothest finishes require wetsanding, but you'll want to make sure that the box is sealed and that the bare MDF doesn't get wet. If it does, it will swell and ruin the entire project.

                              Adding the FG to the box really only complicates things. You would need to make sure there aren't any air bubbles, and once it's dry and cured, you would still need to use filler to get it perfectly smooth.

                              One thing I noticed with your design is you used braces on the corners. On a box this small, that is really not necessary. It appears you've got 1/2" mdf (recommend 3/4", but 1/2" will suffice, but nothing thinner really) and there is enough on the edges to pre-drill and just screw into the edges of the wood. So, I would remove all the little corner braces you have, and that would make sealing it a whole lot easier.

                              Hope this helps a little bit.
                              I cant take anything apart because it is glued. But of course the glue didn't do such a good job with sealing it. I have all the screws couter sinking with the MDF. I think I'm going to just use bondo to make everything smooth and even. Then sand it down like you said.

                              But with the bondo, can i round out the edges with it. Like where the MDF comes to each other and forms a 90 degree angle (|__)?

                              Yes I did use braces. Because I messed up on the one end when a screw separated the MDF when I predrilled and everything. And Im using 3/4 not 1/2. And again, the braces are also glued to the rest.

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