The Fiberglass Box How-To!

I’m assuming you have already weighed the pros and cons of building a fiberglass box. This is a simple how-to tutorial to get started. I based this directly off my 3rd gen hatch well sealed box installation. Don’t expect me to have all the answers.

Things needed:
Latex Gloves (resin burns your skin… trust me)
Glass or Metal Containers (resin will melt lightweight plastic containers, thick plastic might work)
Safety Glasses
Quality Face Mask (Cheap masks do not work, they keep fiberglass out but flumes still get in)
Scissors
Heavy Duty Aluminum Foil
Turtle Wax (works as a release agent)
Masking Tape
Duct Tape
Cardboard
Poster Board
Wood Screws
MDF
Liquid Nails
Bondo
Wide Plastic Rolls (to cover body and any area you do not want resin spills)
Lots of Toss Away Paint Brushes (easier to just throw away a $0.50 brush than clean a good one)
Paint stirrers
Resin
Resin Hardeners
Baby Bottle (easy way to measure out the resin hardener)
Fiberglass Mat
Fiberglass Woven Cloth (biaxial works well)
Jigsaw
Drill
Belt Sander
Palm Sander
Putty Knife
Fleece
3m Glue
Tee Nuts
Machine Screws

1. First step is to decide what you’re going to build. In this project I needed as much room as possible, so my design was to use all the space possible. Your project might be different. Take the time to take some pictures and do some sketches and measurements. More than likely you’ll be putting countless hours if not weeks in on this project and you want to look great once done. I actually took some pictures of my old box and sketched the design the fiberglass would look like. I played with the design a little bit until I came out with the shape and style I liked.

2. Now that you have your idea it's time to start looking for resin and fiberglass. Thing to remember about fiberglass is it’s very strong in curves but weak on straight pieces. Don’t plan on just putting down a couple layers and calling it done. You’re going to need lots of resin if your project is anything like the size of mine. I used around 6 or 7 gallons of resin! This gets very expensive if you’re using the cheap resin from Wal-Mart or another store in Quart size cans. I would strongly consider looking into on-line resources. I bought my resin from www.uscomposites.com in 5 gallon pails. Not only was it cheaper in price but it was a higher quality resin so it lasted much longer. After you get your resin picked out make sure it comes with the hardener, usually it’s included but always check. I use fiberglass mat to make the shape of the box, because it conforms very easy to odd shapes. Also be sure the fiberglass media is compatiable with the resin you picked. Fiberglass woven cloth is usually needed for additional strength.

3. Now comes the fun part! You can start making your design. This is where the cardboard and duct tape come in handy. You need to make tapered walls to hold the fiberglass when you make the box. DO NOT GLASS YOURSELF INTO A CORNER! Take some time to think everything over before you start hacking up cardboard. You need to make sure every piece you tape in has a gentle taper to it and there are no other obstacles in your way. If there is some protruding corner you will have to make a cardboard taper off that. This is going to be the backbone that holds your entire fiberglass while you do the work. Be sure to make it strong enough you can paint and poke it without it falling apart or bending. From the pictures below you can see what I did. All the areas I used cardboard on either had: bad angles, no support, or protruding objects. You’ll also notice I did not cardboard any of my carpet area. This is because it had all the proper angles I desired and I wanted the box to fit tightly against the car once done so it does not slide or rock. Be sure to provide several anchors for the bracing like I did with the duct tape in the photos. Also make sure it’s higher than the box you want to build (like my case around the hatch seal and the area above the gas tank).

4. Now that I have my bracing laid out it's time to go back and masking tape it all off. This means covering all cardboard, duct tape, carpet, and anything else I did not want the resin to soak into. In my project I even covered the area where the hatch motor slot opening is located. Make sure you don’t get too lazy and leave gaps between your tape and your bracing. For my larger areas I resorted to duct tape because I was lazy. Make sure all your tape is liquid tight too.

5. It’s time for the aluminum foil. Cover all your fresh tape in aluminum with masking tape around all the corners. You also want this to be liquid tight. Think of it as two different shells. Hopefully all resin will be contained by the aluminum foil. If something should leak then hopefully your full masking tape layer catches it before it gets to the interior.

6. Next step is to use plastic rolls and cover the area around the car. This is always a good idea, because eventually you’ll get distracted and spill the resin. For me this meant covering the quarter panels, tail light and bumper area, and the interior opening above my bracing where my amps are pictured. Duct tape or masking tape is fine to use to hold the plastic in place. I actually used duct tape because I knew this project would take me weeks to months to fully finish.

7. Time to Wax! I took out the turtle wax and covered all the aluminum and masking tape areas with a thin coat. This will help the fiberglass pull away easier, thus saving some time picking it all off. Be careful not to rip the aluminum foil layer.

8. Now it’s finally time to start thinking about “fiberglassing”. I modify my paint brushes by cutting most of the bristles off only leaving about 2” of length. This helps when working with the mat. I then put on the gloves, mask, and appropriate clothing (long sleeves and jeans). Cut the mat into small squares. I prefer working with squares a variety of different sizes from 3” x 3” to around 6” x 6”. If you’re a beginner it’s always easier using a small square especially if you have to wrap it around a lot of corners. Also there is never enough squares cut out. Every time I had “enough” I ended up running out in the middle of a resin batch.

9. Now it’s time to put to fix the cup with resin. Easiest way is to have a container that provides a line so the exact amount of pure resin is known before adding the hardener. Follow the instructions exactly! Only mix up enough that it can be used up in its life cycle. Don’t try mixing up a complete gallon at once, unless you’re sure you can use it in the time given. Too little hardener and the fiberglass will take days to cure (sometimes they stay flexible even after). Too much hardener and the batch will cure in the cup and may even make the fiberglass brittle! Take the time to use the babies’ bottle to match the resin and hardener up exactly right!

10. Usually a batch has around 15 to 40 minutes life cycle. Better get started! Take a mat square start at the very back or bottom of the project first. For me this was in the very bottom of the well. Put the square down and dip the brush in the resin cup and plop some resin onto it. Here comes the tricky part, work the paint brush into the mat to make sure the resin thoroughly saturates the mat without having any air bubbles. Air bubbles between the bracing or layers of mat make the fiberglass extremely weak! Do not get to much resin on the mat either. If it starts to pool up on top of the mat that means you’re using too much! To fix this either try dabbing the extra off or add more mat to the top. This may take some time getting use to. That’s why I suggest smaller projects first. It takes awhile to get down the proper technique and to figure out how to stroke the paint brush to push out the air bubbles. Generally stabbing motions are better than trying to pull the resin around. Usually when trying to pull the resin around it'll be pulling small fiberglass mat fibers off with it making clumps. Once the mat is free of bubbles and saturated throw another one down barely overlapping it. Do the same with it. Soon you'll find out that it’s harder than it looks. Try not to build past 3 layers deep at one time. After 3 layers it’s hard to push the bubbles out. As you start to grow out to a wider area go back to the center again and work up another layer. You want to make your 3 layers before the resin starts to cure. If it’s curing in the area your working move to a different area. You’ll have to experiment on how fast you can cover areas. If you’re a beginner you’ll only get a very small area in at one time before it starts to cure. Don’t rush yourself and create air bubbles. This project is costing you a lot of money already. You do not want to have to scrap a piece due to air bubbles.

11. When all the resin has been used then either quit and wait for it to cure or go make another batch to finish out the first 3 layers across the entire area. Try not to run out of squares in the middle of a batch. It gets messy cutting with resin soaked gloves. If the full 3 layers are done already take a break and let the project cure before doing any more. Curing time will vary on the resin and hardener amount I found out a day or so was plenty of time. The picture below shows what happens when I tried to do too much at one time without enough resin on hand. Try to avoid letting the fiberglass cure with regular mat unfinished. This makes the project much more difficult to go back and finish now. Also remember if the resin cures then to toss away the brush and start with a fresh one. There really isn’t any point of cleaning these cheap brushes. If the resin starts to gell up, then it either took too long to put on the fiberglass or to much hardener was added. If this happens throw the batch out and start over with a new one. You can not just add more resin to thin it out the mixture. This is a chemical reaction so it's hard to counter act.

12. After the 3 layers have cure totally go back and start laying more fiberglass down. I used mat still due to the fact I wasn’t comfortable enough to try the stiffer woven cloth yet. Depending on the project size you might be able to remove the shell of the box right now with only 3 layers. My project was so large I went 6 to 7 layers before pulling it out of the car. It’s a slow building process. After you’re sure it’s sturdy enough to pull out of the car gently remove it. This is where you find out if you did all your bracing correctly. If you did it should slide out with a little force. Remember it’s going to be air tight fit. Once out I laid it on some saw horses. Working outside the car is more comfortable. Not only do you not have to worry about dripping on the car, but you can maneuver the box around to the area you’re working on. For me since I'm 6+ feet tall I didn't have to bend over as much. This is also a good time to remove the left over pieces of aluminum foil stuck to the cured fiberglass. You can add a few more layers now if you wish. Below is a picture of my box shell outside of the car.

13. Usually by this time you can move on to woven cloth. This type of fiberglass is stronger but generally less flexible for corner. Don’t be afraid to mix several different weights or even types. Depending on the woven media some may contain a woven one-side with mat on the other combo. If yours does not do not worry. You can substitute in regular mat. Mat is needed between the layers of woven cloth due to the fact the woven layers are not suppose to touch. Some woven cloth can not have there stitch seams running the same direction. That means you will have to criss-cross your layers. Woven is a little harder to work the resin through also. With a little effort you will get use to working with it. If you can though save yourself some time and just by a woven/mat combo. I'm leaving this fairly open due to the fact there are many different types of woven media.

14. The overall thickness will depend on the project size and shape. Some people try to put an exact thickness on it such as ½” to ¾” thick. I do not really believe in such measurements. It really depends on the size and shape of the box. If your box has long straight sections like mine you’ll want it thick enough it doesn’t bend. Mine had 6 to 7 - 0.75 oz layers of mat then an additional layer of air craft woven fiberglass which was around 40.5 oz thick. I should have probably done more.

15. After the box has fully cured again it's time to test fit it back in the car. I did this once with the bracing in place then I removed the bracing and put the box back in. After I decided it was thick enough I used the Fiberglass Bondo I bought from Wal-mart to smooth out any bumps I had. This made the inside look nicer and added some overall thickness. I also added bondo to the outside as well to fill in any indents where the aluminum foil folded weird.

16. Now it’s time to figure out the total cubic feet the custom box contains. I went for the “redneck engineering” approach. I filled a 5 gallon bucket up with water and poured it into the box. Approximately 53 gallons later my box was pretty full. I then used: 53 gallons / 7.481 = 7.08 cubic feet so my divider wall will have to be placed to allow 3.4 cubic feet on each side or so. I then made a divider template out of the poster board. Take into consideration how the lid will be formed. After all the edges of the shell and divider will form the lid edges. This is important to get the right look becausse the edges form the slope of the lid. The divider template was used as a barrier when I filled the box up with packing peanuts to find 3.5 cubic feet. After I had found the divider location I traced it in marker on the opposite side. This is where my MDF divider went in my box. If you have one driver you won’t have to worry about this part. If you have 2 or more you will be doing this. Transfer the template you cut from the poster board onto the MDF and cut it out. It might take a few times getting the MDF trimmed properly. I used a belt sander to help fit it. After I was happy with the placement and the upper edge height I used liquid nails to put a thick bead on each side. I had to let that cure for a couple days. Make sure you like the location, because once cured it will be rock hard.

17. While that’s curing you don’t have to just sit around and watch it. Now is the time to make the MDF mounting rings! This is what the sub will be attached to. You surely didn’t think we would just drill screws into fiberglass did you? A MDF ring can be one piece where the sub mounts on the outside. If you do this the sub will be above the box face. In my opinion one piece designs just do not look right on fiberglass enclosures. Making a 2 or more piece ring is actually easier overall when you see the later steps. A 2 piece ring means the sub will screw into the lower ring and the second (top ring) will be a recess for it. This means the sub can actually fit lower into the box. You’ll need lots of measurements. Take into consideration your sub cut-out diameter, outside diameter, and surround gasket thickness. For example if your sub is 1” surround gasket thickness and you’re using ¾” MDF you’ll probably want to use a 3 ring design so the sub face will be recessed ½”. If you’re using 1” MDF this is not a problem you could use a 2 ring design. See how the gasket thickness and MDF thickness makes the ring design change? Mine allowed me to use ¾” MDF and a simple 2 piece ring design. Don’t be worried over ¼” or so difference that will get taken up by fiberglass later. Note from the picture that the inside diameter of the top ring needs to allow enough room for the sub to slide freely into it. This means you’ll need a couple extra inches worth of matter on the bottom and top rings so you can attach them to each other. You will also want to take into account if you plan on using grilles too, because that will extend the mounting area needed out even further. You now will have to scribe out your lines. I used a board attached to the middle by a nail. I then drove extra nails into it at the radius I wanted measured off the nail on the end. That way I could simply spin the board around and get a perfect circle to cut on. After there cut out you’ll want to save the left over pieces and test fit on the sub. To make sure it fits properly, some sanding may be required. After you’re happy with the designs pre-drill some holes into the outside and then apply liquid nail to them. I used nails in 2 of the 4 holes, so I could realign the rings back together and sink your wood screws into all the pre-drilled holes. End product will look something like the picture below. Now it’s time to let these cure up.

18. Now that the rings and box has cured. It’s time for a test fit. Here’s a picture of my sub in the box. Not only will this give you bragging rights on your forum, but it’s an important step in the project. You need to figure out where to mount the ring. In my application too high and it would hit my hatch trim (take into account for excursion too) and too low and the sub would rub on the box and not seal properly.

19. After you have decided on placement slight sanding might be necessary to get the rings to fit right. The only limit is how much work you want to do. After my rings were fitted right. I took several measurements off the sides of the ring to the box and recorded each. These will be your support braces to hold the rings up. Some people just mount the subs off the face/top of the fiberglass lid but in my opinion that adds additional stress. By using bracing you not only remove 20 to 30+ lbs off each side of the lid but you add additional strength. Think of it as a MDF backbone. My pieces were about 1” to 1.5” in width. Each ring received 5 to 6 supports. These are pre-drilled then attached with nails to the mounting rings for the test fit. Place the rings back in your box with the supports attached carefully put the sub in. This is to make sure the sub does not sit too high or too low in the box. If it does modify your ring support lengths till it’s just right. After you’re happy you can use wood screws in place of the nails. Next step is liquid nail them firmly into the box. I used liquid nail on both the bottom attachments and around the ring itself.

20. Additional bracing can be added also. I eventually put in extra MDF braces that attached off the supports to the sides of the box. This made the box even stronger. All these braces were measured and cut to fit. I used wood screws to attach them to other braces and liquid nails at both ends. Once I was done I had a MDF skeletal structure.

21. Next step for my project was to provide clearance for the hatch shocks. From my box design my hatch shocks would have hit off the lid. This is where fiberglass is so nice to do custom work. I bought some 1” wooden dowels from Wal-mart. I then cut them to fit and aligned them to where the hatch shocks would recess into the lid. The picture below shows them in the box. I eventually mounted them on wood screws that I drilled directly through the box. The punctures were the areas that needed to be trimmed back so I had no problems.

22. This is one the more personal rewarding steps. It’s finally time for the fleece wrap. This will give you a general idea what the box will look like once done. I used white fleece because: it was cheaper and it’s easier to see resin on it. To get the fleece to work properly spray the surface down with the 3m glue. Start in the middle and work out from that. For me this was the divider wall to rings, under the dowels, and then finally I started to pull the fleece tight and glue it to the outside of the box. Make sure your fleece sticks! Trim the excess off the box with scissors. Take time to marvel at your great project.

23. Now it’s time to mix up a batch of resin. You should be a pro at this by now. Make sure to saturate the fleece thoroughly. This stuff sucks resin up like crazy. Don’t worry it’s worth it. Make sure to get the ring surfaces well this is going to be the base for your fiberglass lid. There is no need to cover the empty area out in the middle of the ring. Do not skim past this step, its important! If you do not get a proper fleece attachment layer it will be much harder later.

24. Once that was fully cured I removed the screws for the wooden dowels and pried them out. In hind sight I should have waxed them first.

25. I cut the extra fleece away from the mounting holes with a putty knife and test mounted it. Also note the cardboard bracing on the corners gave me extra room for the fleece wrap over! By this point of the project I was getting extremely excited as to it was finally taking shape.

26. Now is time to do more fiberglassing. Those left over center pieces said to save from the MDF ring build. They come in very handy now. Take the top ring cut-outs and drop them back into the box. Cover the gaps with masking tape and wax. Now you can start out with mat again. It is a good idea to get the 3 base layers again. After that’s cured move to your woven stuff. The lid in my application didn’t have to be super strong because of the ring sizes, but still the stronger the better in my opinion. Keep building until you’re happy, but be sure not to use up to much room and cause clearance problems.

27. Now that the lid is done. It's time to smooth it out with bondo and make it look all pretty. Not a hard process: mix, apply, cure, sand, and repeat till happy. Notice how all the curves of the box come out after the bondo was put on it. You can also do spot putty at this time if you wish.

28. Hurray, now it's time to drill out the mounting rings and grilles. Just don’t mess them up. Put the sub in how you want it facing and mark the holes (pay attention to the logos). Take the sub out and drill the holes out to the size of your tee nut shafts. Be sure to actually drill into the mounting rings not the fiberglass. What is a tee nut? It’s a metal insert used in wood so fasteners can pull off metal threads instead of wood. You might as well protect your investment and use them! If you’re tempted to just screw them into the MDF think again, over time the MDF will crumble from removal and normal wear and tear. After your holes are drilled put some liquid nail on the tee nut and press them into place from the underside of the box. Let that cure. You can also mix yourself up a bondo milkshake if you’re worried about air leaks. A bondo milkshake is bondo and resin mixed together in a 2:1 or 2.5:1 bondo to resin ratio. That’s how I got that pink color inside my box if you were wondering. This can be painted on with more toss away paint brushes. Not only does this fix air leaks but it adds additional strength to the walls and braces. Make sure to get the underside of the lid too! As this will add a slightly better anchor for the lid to rings.
Make sure to do the bondo milkshake after the tee nuts. The bondo will be to hard to press the pronged tee nuts into. So, it'll be a time saver over trying to rip off the bondo in the mounting spots.

29. After everything is cured drill your holes for your wiring. This would be the step where you paint or cover your box. Finally silicone up all of your wiring holes after the wires are through and cover all the tee nuts with a thin layer of silicone this helps make sure there are no air leaks.

30. After proper curing time for silicone you can stuff and install the subs. I usually wait 48 hours. This allows for the gases released by the curing phase to escape before putting hte subwoofers in. Supposely these gases can eat away at the rubber surrounds. Now you finally get to hear what your new box sounds like. Just sit back and think about all the money you saved doing this yourself over paying a shop to do a custom.

Site Index:
Page 1 - Introduction
Page 2 - Current Project To Do Lists
Page 3 - The Interior
Page 4 - 2 18" Kicker particle board box (old design)
Page 5 - 2 18" Kicker fiberglass box custom
Page 6 - General Mods
Page 7 - My Z20 Special Appearance Package
Page 8 - My fiberglass non AC heater box mold (1LE version that has been discontinued by GM)
Page 9 - New High-PO 383 in the build
Page 10 - Glasstek Hood Repair
Page 11 - Fiberglass Box How-To <-- You're here
Page 12 - My Xbox Game for 1¢ I bought from the store with proof
Page 13 - SOLD! Twin 62mm T4 Turbo Kit From BBS DESIGNS for 1200 HP! SOLD!
Page 14 - To Do List before motor completion