I'm preparing till fabricate some custom speakers pods for the A-pillars in my vehicle. I plan on using the existing pillar covers and building off of them to mount a 3" mid and a 1" tweeter, as opposed to creating a mold and using it (unless that would be better). I'll be using 1/2" MDF speaker rings, mounting them with dowels, then stretching fleece fabric around the part to create my base.

I've watched countless videos on how to add pods to existing parts and also how to create subwoofer enclosures in trunk cavities. I feel pretty confident I have a grasp on most steps. However, there are a few things that are still not clear to me:

1) When is it necessary to use a mold release agent? Most people use masking tape then apply resin then start applying their glass layers.

2) What is a gel coat and when would I use this step?

3) I've also seen people use foil to conform to the panel/part/object, then apply resin and glass. When would I use foil rather than tape?

4) Some people apply a layer of Bondo after glass then sand, prime, and paint while others paint the glass (no Bondo). Is Bondo used because it provides a nice smooth finish for painting, or is it to add more strength? Both?

5) I will be covering the pillar covers with headliner fabric when all is said and done. I'm concerned glue won't stick to the back of the pillar covers since they are smooth plastic. Will the fabric glue adhere to the resin/fleece layer I will initially be applying? It will wrap around the back a bit. I wouldn't think staples would work since the plastic is so thin.

 

1. You use a mold release agent when you are using a mold and don't want your part to stick.

2. You will not be using gel coat. That is done when using a mold. With moldless construction you will paint the outside of your part.

3. Foil and tape are used in place of mold release. If you want your fiberglass work to hopefully stick to the plastic trim piece you won't use any of it.

4. Bondo or other filling compounds are used to fill voids or boo boo's. If your fiberglass work is good you won't use it.

5. Not only fabric glue but you also need to be concerned about how the fiberglass will stick to the plastic trim piece you are using for a base. Sticking fabric is an easy application. There are many glues you can use that will work well.

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Have you looked at what a 1/2" thick MDF ring for your speakers looks like? I would not use MDF and just mount the speakers directly to the fiberglass. Your A pillar speaker housing is going to be chunky enough already. You don't want to add even more stuff to bulk it upt.

 

I'm not sure how else I would make the shape I want without using speaker rings and dowels. I could use 1/4" plywood. Doesn't have to be MDF. I don't want the pillars to looks chunky, so I'll have to get creative with speaker placement. Here's an example of what I'm aiming for.



Regarding the fiberglass and glue sticking to the fabric, this is why I'm wondering if I should make a mold/copy of the A pillar cover and use it instead so it is all fiberglass and no plastic to contend with.

 

Just asking the obvious, have you looked to buy a set for the car or even buying something then modifying it to fit your A-pillar.

In all the fabrications I do it's always easier to start with something and adapt it vs creating from scratch!

 

Yes, I've looked, but most things I found are too large/bulky. I want to do fully custom so I can make them exactly how I want.

Sorry if I was confusing in my original post. In addition to my specific questions about the A pillar project, I was asking general questions about using fiberglass to make enclosures in vehicles, whether it be a subwoofer enclosure molded to the side of the trunk or custom speaker pods in the kick panels, doors, and A pillars.

 

I like to do what I call lost foam method for fiberglass. I make a positive mold in the shape of the part I want using foam and modeling clay. Then lay the fiberglass on top. After it's cured you scrape the foam & clay out from the inside. Then sand and paint the outside. This is where the bondo you asked about comes into play. Usually there is some air bubble crease or divot you want to fill and bondo makes it easy.

With this method you can form the round mounting location for your speakers. Then lay up enough fiberglass so there is enough bite for your speaker mounting screws. That way you can make your fiberglass fit as close as you want to your speaker for a sleeker look and run your mounting screws/bolts into the fiberglass and forgo using an MDF ring which will just take up space.

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If you will be using polyester resin make sure you use compatible materials for your mold as the solvent in the resin will attack many foams and plastics. Finishing polyester resins contain waxes that migrate to the surface during curing and must be removed with solvent and sanding before applying another layer. Layup polyester resin doesn't have wax but cures with a almost sticky and rougher texture that's ready to receive another layer without prep. Then there are epoxies, which are my favorite though more expensive. You can use any type of foam you want for a mold and don't have to worry about the resin eating it.

 

Thanks for the thorough explanation. Glad to know I have options. I may try both and see what works better. I bought two extra pillars just in case.

 

I don't think I mentioned that the A-pillars are already covered in a thin headliner type fabric, so I'm wondering what glue the OEM used.

 

In the auto industry it's hard to tell. They use many custom products that aren't commonly available. You will need to remove the old fabric and glue, either by sanding or with solvent, before fiberglassing to it. Some plastics don't bond well with polyester or epoxy resin so you will want to stack everything in your favor by preparing the surface well for bonding, including a nicely scuffed/sanded surface.

For installing the new fabric I would use a spray contact adhesive. I've had good luck with 3M Super 77. If you use a fabric that can be stretched or deformed I would cut your fabric over size and roughly form it to the shape of your piece. Then spray the back of the fabric and your A pillar cover with adhesive. How much "working" you can do depends on how soon you start sticking the pieces together. For this I would start applying the fabric as soon as you are done spraying and while the fabric is wet. Working when it's cold and humid will help slow the drying of the contact adhesive and buy you more time but you will only have a minute or two. If you follow the can's instructions and wait for the glue on each piece to mostly dry then they will stick instantly when they touch and you might not be able to reposition or stretch the fabric.

I've also had good luck using carpet and VCT adhesive intended for floors. Apply it to only the fiberglass part. It gives you more working time but it doesn't have much high initial grip so it could be harder to stretch your fabric and get it to stay where you want. If using this method make sure you don't use too much adhesive. If you use too much it will saturate the fabric and bleed through to the surface.

 

Thanks for the tips! I'll be sure to practice on a sample piece before diving in to the main project.

 

I have the pillar covers off and it looks like they used some sort of heat crimp tool to secure the fabric along the one side, but there are no crimps along the opposing side. Guess they used glue. Anyway, I'll follow the advice given here and hope for the best.

 

I wonder if I could just stretch the fleece around the pillar cover and secure it with clothes pins on the back so it stays tight, then apply the resin ensuring it wraps around the back a bit. Then once it hardens I should be able to remove the clothes pins, trim the excess fabric, and the form should hold its shape. Does it sound like this will work? I don't think staples or glue is going to work/stick to the back.

Maybe a plastic welder:



or a clamshell sealer. I could disconnect one of the heating elements so only one side would heat/weld:

 

Both tools worked! Fabric holds nice and tight. Better than any glue could do I'm sure. The clamshell crimper is more appropriate because it doesn't get as hot, has a hi/low setting, and the crimping action is convenient and results in a stronger bond. I disassembled it and disabled the bottom heater so only the top gets hot.

 

Looks good and professional.

 

I'm having difficulty sourcing a knit fabric that matches the existing material in the vehicle, so I may go with vinyl instead. Unfortunately, my heat crimp tool won't work with the vinyl, at least I don't think. I have a can of 3M 77 contact adhesive, but I don't think that will be strong enough. Any other ideas?

I'm nearly finished with the pods. Just need to touch up with some glazing putty, sand (again), seal the primer (and upholster obviously). Turned out very nice.