Adventures in fiberglassing: custom fabricated tweeter pods

[This project is more than a year old, but I never got around to posting anything earlier.]

I generally like to make things (I used to say “build” things, but that was misconstrued by some manager/academic types, who apparently have a very different definition of “to build”), whether it’s software, writing, or “hardware”.  I usually talk about the first, but I occasionally do the last (much to the dismay of my wife, who has nonetheless been very patient! :).  I also like to try new things–I probably care more about the process and experimentation, learning what’s possible and how to do it, that the final product (which is not to say that I don’t care about the final product at all, but it get’s boring pretty quickly for me). So, sometime last year I decided to upgrade my car speakers (I also Dynamat-ted all doors, but I didn’t take photos of that adventure; one tip, though:: make sure you sit down properly, because after crouching down on tiptoe for almost an entire day, I needed physiotherapy for my heel tendon :) —now my Subaru’s doors sound like a Mercedes when you shut them). However, the new tweeters were much larger than the factory-installed ones, so I took the opportunity (excuse?) to learn fiberglassing and make new tweeter pods.

Let me set the mood by starting with my outfit: I started with the one on the left, but after plenty of PVC dust, fiberglass dust, and acetone fumes, I upgraded to the one on the right. A proper respirator helps a lot, especially if you’re working indoors. And don’t skip the safety glasses (even if you’re wearing vision glasses, as I found out).  Always take the proper safety precautions.

outfit0 outfit

I did not build the entire sail panel from scratch, of course (didn’t have a 3D printer back then but, even now that I do, decent 3D scanners are still not cheaply available, although that’ll hopefully change soon).  I modified an OEM sail panel (I got the cheap ones, that aren’t designed to house factory tweeters, since I was going to cut them up anyway).  The tweeter pod was fabricated from a piece of common PVC pipe.  Cutting those can be a bit messy, but the Dremel is a fantastic tool. If you’re thinking of fabricating anything and you’re going to buy just one tool, it should be a Dremel.  In fact, just stop reading and go over to Amazon and order one now! [No, Bosch is not paying me for this]

I used a cutting disk for the PVC pipe. Go slow to avoid melting the PVC too much and cut the piece a little longer, then sand off the molten bits of PVC (if any). This will give you a nice, clean edge.  Make sure to use a sanding block (or put the sandpaper on a flat surface) to get a good edge.  I used masking tape to mark the cutting path; getting this straight saves you some sanding.  Also, instead of going completely freehand, I like to put both the tool and pipe on a steady surface, and then just slowly roll the pipe until it touches the disk; makes cutting much easier to control. I also notched one edge with a sanding band (which is what is attached to the tool in the picture on the left), ostensibly to route the wires, but that turned out to be unnecessary.

For the OEM sail panels, I used a cutting bit and did freehand cutting (picture on the right).  You don’t need to be terribly precise anyway.  Make sure you move the tool in the right direction, otherwise cutting will be difficult and the bit may kick back (although, even if you gouge the piece by accident, it’s not a big deal at this stage—and then you’ll know which direction to move along :).

pvc-pipe dremel-cutting

The PVC pipe inner diameter turned out to be a tad small, but the Dremel with a sanding band was enough to fix this.  Again, I put both the tool and pipe on a steady surface, and just rolled the pipe around the spinning band.  You can also use a prop on the outside part of the pipe, to make sure you get a (relatively) consistent depth, if your hand isn’t particularly steady (mine isn’t, but you can always devise a jig or prop to compensate).  The first picture below shows the original pipe (I used the two scrap pieces for practice) and the thinned down pieces are in the sail panels.

As I mentioned earlier, this does not need to be a precise job, since everything will be covered in fiberglass, eventually.  However, you want to make sure that the tweeter pods (aka. PVC pipe) are aimed in the right direction.  You want to go down to your car and test, then use a pen to mark the pipe around the edges where it touches the sail panel hole.  Then go back and hotglue the pipe temporarily in place.  This should be strong enough to prevent it from moving, so you can go back to your car again and double-check (and triple-check) the aiming.

It’s crucial to get this right, as you obviously cannot re-aim later.  So, measure twice (or more)!  Also, it helps if your tweeters are already broken in when you do the aiming. Mine were pretty harsh for the first couple of months (to the point that I was wondering how reviews could be so good), but eventually became pretty mellow.  I did all this just a couple of weeks after using them in temporary mounts (I thought this would be enough to break them in; apparently I was wrong).  Now I’m pretty happy with the tweeters’ sound quality, but I would have aimed slightly differently if I had done this later (still not too bad, though).  So, if you don’t mind your car looking like a wreck, take your time breaking them in before you do all this.

pipe-on-sails-1 pipe-on-sails-2

pipe-on-sails-3 pipe-on-sails-4

Once you have the aiming down, it’s time to give the sail panel it’s final shape.  You do this by stretching something that can absorb the resin over the frame that you’ve essentialy created.  A common material for jobs like this is speaker grille cloth, which you can get pretty cheaply on eBay or Amazon.  Another popular material for bigger surfaces seems to be a fleece blanket.  However, for this small job, the speaker grille cloth is perfect: it’s very thin, it’s quite strong, stretches nicely, and follows the contours well.  It’s also pretty absorbent (enough for the glue and resin).

You will need to superglue the cloth on the frame, but you’ll need activator to make it work. You can usually find a package that has both the cyanoacrylic glue and activator at any hardware store.  By the way, this works great for any hard-to-bond surfaces (not just speaker cloth on plastic :).  The bond may not be strong long-term, depending on the materials, but in this case it doesn’t matter (it’s pretty good actually). Make sure to stretch the cloth enough to get rid of any kinks.  The good thing about speaker grille cloth is that it’s pretty stretchy, so even if you have kinks after you’ve partially glued some sides, you can still get rid of them by just stretching harder. You may need a bit of practice doing all this, as the activator evaporates quickly, but the glue sets almost instantly if it touches it. So, you may want test in which order you apply glue and activator, but it’s not very difficult to get right after a couple of trials.  Plus, you can always rip it off, scrape the glue, and start over.   Once the bond is good, cut off the excess cloth with a sharp craft knife.

speaker-cloth-1 speaker-cloth-2

Now you’re ready for the actual fiberglassing!  First, you apply some resin on the speaker grille cloth, without any fiberglass mat. In my case the first layer I applied was too thin (you can clearly see the speaker cloth), so I went for one more layer before the mat.  You want it to be nice and thick (to fully cover the cloth), but not too thick (you can always sand it down, but resin takes forever to sand by hand if you overdo it).

resin-layer1 resin-layer2

After the first layer has cured, you can move on to the second layer, with the fiberglass mat.  First, I sanded down the resin a little bit, and cleaned off the wax release agent (apparently resins contain a small amount?).  Then, place pieces of fiberglass mat and dab them with resin on a paintbrush (I used a cheap $1 paintbrush, 1/2in wide—you may want to pick up a couple of those, resin can set fast, especially when you don’t know what you’re doing in the beginning).  One thing I found out the hard way: don’t be lazy and use a stick to properly mix the hardener into the resin, not your brush!  Otherwise, the brush will absorb the hardener (capillary action), which will have two undesirable effects.  First, the resin on your brush will set and render your brush useless.  Even worse, the resin that you put on the piece won’t have enough hardener, and it will take forever to cure.

first-sanding mat-layer3

Once the resin sets, one more round of light sanding and cleaning off the release agent.  If you have excess resin around the pipe edges, you can also use the sanding band on the Dremel to shape it a bit (as I said before, cured resin is pretty hard and large pieces take forever to sand down by hand).  Take it easy and slow with the Dremel though.  Also, make sure to wear your respirator (fiberglass dust is particularly fine and gets everywhere).

You can do another mat layer, if you want, although that much strength is probably not needed here. Finally, you’re ready for filling. I used common Bondo body filler from an automotive shop.  This thing cures pretty quickly, so make sure you mix just the amount you need.  In my case, I thought I’d mix more, but my first batch hardened before I even  got to the second pod. Admittedly, I was also pretty slow spreading the filler, since it was my first time. However, filler is pretty easy to sand, so go ahead and make a mess (within reason).

second-sanding bondo-filler

Once the filler sets, you’re ready for the final sanding.  It’s crucial that you use a sanding block, to get a nice surface shape (don’t use handheld sanding paper, or you’ll get a funny surface with grooves etc).  Do the usual progressively finer grit sanding.  I also used wet sandpaper at the final stages. Finally, spray paint everything (I used gray primer then black paint that matched the OEM trim). Needless to say, don’t do this indoors, unless you want graffiti on your walls (I guess you could use plenty of sheeting and open windows, but it’s just easier to go outdoors).  I used a skewer taped to the backside to hold the pieces while spraying. I also covered the foam strips pre-installed on the sail panels with masking tape.  Follow instructions on paint bottle for drying times, etc. This was also my first time using canned spray-paint, but no surprises here. Just take it slow and don’t start spraying too close (you can always move closer and/or spray on more paint, but not the opposite–however, this is just common sense).

So, after all this, here is the final result.  The picture on the left shows the OEM sail panel for factory tweeters and my hand-made, fiberglassed sail panels.  The picture on the right shows the sail panel and tweeter installed.  If you look closely, you’ll see some small surface imperfections (I think it’s because I didn’t use a sanding block on the last, light pass with fine-grit sandpaper–it was getting late and I was getting tired and impatient), but not too bad for a first fiberglassing job!

handmade-vs-oem installed

Postscript: Of all places, I got excellent fiberglassing advice at… an NSF review panel, from a university professor who turned out to be a machinist and body shop worker (maintaining an antique car collection) in his former life! Wow!