This article written by: Toby Broadfield
This article will show step-by-step process of building fiberglass molded panels from the skeletal beginnings, to the final painted product. Keep in mind that a painted product is not always the route taken. Some customers want it vinyl wrapped, carpeted or wrapped in something else…. carbon, Kevlar etc. Using automotive paint as a finish is obviously the most time consuming, considering it has to be perfect. I will update this as I see fit. I am sure I left some things out that might help you. There are obviously a lot of things I purposely left out, but I wasn’t going to go over EVERY step and detail.
Skill Level: 5 of 5
Time Required: From hours to months. The size of the project will determine how long it takes to complete.
Tools Required: Air sanders, die grinders, body filler shapers. The list is nearly endless. If you are taking on a project of this magnitude you should research what tools are required for your personal project, and skill level.
This is a basic guide to help beginners get an idea what is involved with building custom fiberglass interior.
The most important step is your frame work or “skeleton” as I like to call it. This is what you are going to stretch material over to get the shape you want. So you really have to plan out and envision how the material will stretch over it and what shape you are going to have once the material is stretched. You need points to staple or glue the material to and then stretch it to the other sides and staple or glue it over there. Let’s start off with some pictures so you get an idea and we’ll dive in from there. By the way, this is going into a 2003 RX-8……. so it is very limited on trunk space. This customer wanted the sickest looking install in town, so that’s what I gave her.
First off the skeleton:
This is mocked up outside the car after I made sure it fit in the trunk. She wanted (4) 8″ sub woofers and 3 amplifiers. Give it a nice look without too much stuff crammed in there. This is just the beginning of the skeleton work. I still need to fasten on stretch points for the material.
This picture shows the pieces over the amplifiers which are my stretch points for the amp rack. Material will be stapled to the bottom side of those rectangular cutouts, and then stretched to the outside edges of the amp rack.
Back in the trunk to make sure I like what I have so far.
Next I use speaker box carpet to stretch over the amp rack. It is very important to stretch it as tight as possible. This will give you consistent contours, and also prevent it from sagging too much once the resin is laid on. This picture is actually after I poured resin on it and trimmed the edges with an air sander.
So the steps would be:
1) Staple material (carpet, fleece, grill cloth etc.) on one side of skeleton.
2) Stretch tight as you pull it to the other side and staple.
3) Work your way around the skeleton stretching as you go and making sure it is tight every time you lay a staple. I use a staple every 1/2″ or so, or more where necessary. On this particular piece I actually stapled in the amplifier openings first, then stretched to the outer edges of the rack. You will quickly figure out the best way to stretch and attach your material once you mess with it a little. It will also depend on what type of setup you are stretching over.
4) Once all of the material is tight and you are happy with the shape, it is time to resin.
5) Mix up a batch of resin and use a cheapo brush to apply it onto your piece. It will soak up a lot, so don’t be afraid to use a lot.
6) Once it is rock hard and dry, you can use an air sander or grinder to trim the edges clean. I go right through the staples and all.
If needed, you can add fiberglass mat to the backside for extra strength. You will have to decide if your piece requires this or not. You can do as many layers as necessary. I added multiple layers of cloth on the exposed areas on the inside of the enclosures. You can also use this method to seal up gaps, holes etc.
For this install, I needed to get the amp rack to pretty much the final surface grade so I could figure out exactly where the sub enclosures would sit. They hang over the amp rack on each side, so I had to know my final thicknesses so I could continue.
So I sanded the rack down with 80 grit, then sprayed the entire thing with spray bondo. If you have never used spray bondo, it is “the bee’s knees”. You simply spray it on with several coats and let it dry to a rock hard surface. The nice thing about it is that it maintains all of your contours on pieces that are like the ones in this install. If you use standard bondo with a spreader or even your fingers, you lose some of your perfect contours if you get it thicker in some spots etc.
These will come in handy for sanding and shaping. The more you can do with air tools, the better.
This is now sanded with 80 grit and test fit back into the trunk to start the sub enclosures.
Now it’s time for the enclosures. We need to work on stretch points around the edges of the enclosure. You will notice the one on the bottom which has a nice flowing radius. I also had to add ones on both sides. The side that meets the amp rack is just a straight up and down piece. The side that meets the back trunk panel needed a step in it. These don’t have to be 100% perfect since you are going to be stretch fairly thick material over it. Also, you can add to it later if it doesn’t fit exactly the way you want it…… which I will touch on that later.
Now it’s time to stretch the enclosure. However, this one is much more difficult. Part of it has to be stretched to the vehicle itself for that perfect fit. So I staple to the bottom and up both sides since those are all stretch points on the enclosure. I then take the material that is left up top and sandwich the rubber boot over the top of it pinching it into the trim. I then slit the carpet over the sub cutouts and stretch it down into the hole as I staple it around my trim ring.
I also had to make a temporary stretch point on the underside of the trunk ledge. I used it to take out the slack between the sub enclosure and the rubber trunk gasket. This is a picture on the passenger side for reference.
Next step is to resin that and trim it so it fits the way you need it to.
All trimmed and back in the car for a test fit.
It had come to my attention that I didn’t get both sides stretched exactly alike. I had a bulge in one area that I didn’t on the other enclosure. That’s the nice thing about working with fiberglass. I just cut it out and put a patch in to get the contour that I wanted.
Next up is the rear trunk panel. It is plastic, so resin will not stick to it…. which is good in this instance. I glued carpet onto it then did the resin trick on it. It had some dips in it because of the huge cavity that was in the panel for the jack etc. So where the carpet dipped down after the resin, I had to add a bunch of bondo to get it nice and level. I then popped it off the original for a perfect duplicate of the original. In some cases, depending on your job, you will leave your original plastic piece in there and not separate them.
Next up, is sanding the sub enclosures.I also used a nice ring of bondo around the mounting rings to insure I had a perfect contour around the subs.These had to be sanded with basically one finger.
All that bondo just to fill in these little areas,
Now I pre-fit every thing again. Now we are starting to see what this thing is going to look like.
I also decided that it would be stupid to leave the carpeted trunk lid cover original. I didn’t like the idea of opening the trunk to see a bunch of gloss black fiberglass work, only to have a carpet piece overshadowing it all. So I decided to use the resin over carpet method on the trunk lid panel. Simply resin it to get it hard, then the usual bondo and sanding to get it ready for the same black paint. Note: As you can see in the picture, I took the trunk lid off the car and then did the resin job. This way the piece wouldn’t warp on me if I took it off the trunk 1st. Once the resin starts to set up it will get very warm. So the piece will actually distort as it sets up if it’s just a free floating, thin part. The sub enclosures and the like are fine since they are stretched tight and bound by the skeleton.
Now it’s time to get the enclosures to fit perfectly up against the rear panel. Sometimes it’s impossible to get your piece, in this case the enclosures, to fit perfectly just from the initial stretch. So you can go back in and fill or take away material. In these pictures you can see the gaps or where it was impossible to have a smooth transition when I stretched from the rubber trim to the skeleton of the enclosure. I used a product called Kitty Hair to fill in the gaps. It is bondo with fiberglass strands mixed in. So the outcome is whatever shape you want, but with a lot better strength than regular bondo.
Once that dries, you will obviously sand and shape the Kitty Hair to the exact shape you need. Once everything is sanded down with 80 grit and there isn’t any huge gouges etc., it’s time to spray bondo the rest of the pieces.
Trunk lid panel prepped with 80 grit, then spray bondo. Keep in mind that since this piece is out in the sun, it is getting warm. So once the bondo is dry, you would want to take it back inside and fasten it to the trunk lid before it cools off. This will insure it retains the proper shape.
The rear panel and the enclosures after spray bondo.
After the bondo it’s time to sand all of it down to 80 grit again. You also want to make sure that as you sand you rub your bare hand across the surfaces to make sure there are no minor waves. You won’t be able to see them, but you will be able to feel them. Because once it is painted with base coat/clear coat, you will see every little wave etc.
At this time I sealed up the back side of the enclosures using a combination of MDF and more fiberglass cloth and resin. They need to be 100% sealed. I also used just straight resin to pour into gaps etc.
I also had the issue of my rear piece flexing out in the sun when it was drying. So it didn’t clip to the back edge of the trunk around the latch area like it originally did. So I decided to kill two birds with one stone. I mixed up a really “hot” batch or resin…… so lots of hardener. I used fiberglass cloth to add strength 1st of all. And since the resin gets really hot, it would allow me to bend the piece back to the exact shape I needed. The nice thing is that the resin gets non-tacky to the touch, but is still really hot. So at that time I installed it back into the trunk and temporarily held it in place with whatever means possible. I just had to make sure it was exactly where I wanted it as far as shape and placement. Once the resin fully cures, you can pull it out and it will now keep the shape you want.
Up next, the final piece to the puzzle…… the center section that connects the enclosures. Keep in mind that all of these pieces are separate just so you can get them in and out of the trunk. There is no way this could have been done as one huge fiberglass monstrosity. I used a thin piece of 1/8″ hardboard as a starting point. It’s strong and flexible. That way it will bend around the front of the trunk opening and follow that contour.
Making this piece had to be last because I needed the enclosures to be at their final finish grade. This piece is going to lip over the front edges of the enclosures. So I want a perfect seam there…. no gaps. I applied masking tape around the area where I would be spreading in Kitty Hair. It won’t stick to the tape, so it works great and has a really minute thickness. So it won’t throw off the gap between where the center section meets the enclosure.
After I rough sanded the Kitty Hair down, I took one solid piece of fiberglass cloth and laid it over the center section. Resin this and now you have a really strong piece. The back sections of the Kitty Hair are the only things that really had to stay the way they were. This is the only part that touches the enclosure and had to retain perfect gap tolerances.
Sand and trim the edges for a nice finished piece. This piece was simple enough that spray bondo was not required.
Once you are 100% happy with the way everything fits, it’s time for primer. If you are working on little projects, like gauge panels etc, then I would just use spray can primer. In this case, spray gun is the only way to fly. You can also get a lot higher build out of it and it will easily fill pin holes etc.
Now it’s time to sand everything down with 240 grit. A really cool trick, no matter how big or small your piece is, is to use a guide coat. Simply take black can spray paint and pepper it over the entire piece you are sanding. As you sand it will take off the black. It’s not good enough until the black is all gone. This way you can identify minor low spots because there will still be black left in there. You will also not keep sanding in the area once the black is gone. You have to try it to see how well it works. If you find any little imperfections or pin holes, now is the time to fill those with thin bondo and sand with 240 grit.
After that I used spray primer again then wet sanded it with 400. You want to make sure you don’t break through the primer. It can cause goofy things to happen with the base coat once it is sprayed on.
I used the same technique of the guide coat again. After this is all done, it’s time for your final paint. Note: You need to make sure the piece is absolutely perfect
I had my friend paint it since I don’t have an ultra low dust free environment. He runs a body shop, so it was just too easy to take it to him.
This where all of your hard work pays off…… finally assembling it all in the trunk for the finished product. Unfortunately it’s black, so it’s hard to pick up all the details in pictures…… but you get the idea.
This article written by: Toby Broadfield