Hey all, I have been working on a glass box for the wheel well of my civic. The box is for a 15 inch Fosgate Power 2 sub. I've got almost a gallon of resin used in getting the box this far, and I am not having the best of luck. A decent (3yd?) amount of matting has been used as well, and I checked the volume to approximately 3 feet. I'm wondering how much resin a box of this size should take, because I am considering spending another 50usd on another gallon and using more...

Also, how excellent should the box be? Obviously, I'm doing the best that I can but there seems to be air between some of the layers of glass matting. The box flexes a tiny bit when you push on the bottom of it, but not much. My problem is that when I paint on the resin, after a while my brush starts to pick up the matting and make little mounds of resin/matting fiber/air that cause problems. Anyone have tips on dealing with this? Thanks

Hhmm sounds like you've got yourself in a bit of a mess there. The only reason I can think you'd have air in between the layers is because you're not waiting long enough before laying the next layer and also maybe not 'smoothing out' the layers properly using a hard roller. Remember that you need to puch the resin into the matting, not just brush it on.

I'm not sure how much a gallon is in terms of litres but it sounds a lot. You may need to start again

Post up some pics,, not so that people can have a go at you.. just so that we can give some ideas on whether its worth going ahead with the current effort. Air bubbles means it pretty hopeless I'm afraid, depending on how bad it is of course...but the fact that its flexing means its probably not worth going on with this one. Oh and a good idea to use a seperate brush for each layer

for reference, my box used 4.5 sq.m (approx 5 yards) of glass and 4.5litres (1 gallon) of resin. It is approx 37litres.

why dont ya get polyester resin. its about $5 -$15 australian a litre and its a lot easyer to use,.
if you go to a boat building place they should give it to you for about $5-$10 a litre.
i bought 20 liitres with proper rollers that boat builders use and about 10m of matting, and hardener for about $200.

If it flexes,then you will be losing energy with the sub installed. Add either bracing internally (some dowel or rope and add some fibreglass over that) or add more layers of fibreglass (or better yet, do both).



You shouldn't need to "paint" on the resin, and when you are adding the matting, don't paint it on as you will bunch up the fibgreglass and get the problems you are referring to. Instead use a stipling motion (i.e. stab the fibreglass with your brush instead of moving it side to side), this will avoid air bubbles and will avoid bunching up the matting.

besides - getting itchy is all part of the fun

as for cutting yourself?????

You can stop the itchyness by wearing long sleeves and by covering your arms and hands with talcum powder. After grinding, wash your arms and hands and you won't get itchy. Always works for me.

For large air pockets drill into them and fill them with resin before the next layer goes on, if the pockets are too small to drill they are small enough to not matter all that much.

You have the luxury of not needing "perfect" engineering, you can glass in wood bracing in large flat areas where the glass is weakest.

Wow, thanks for all the replies. Talcum powder is a good idea, I'll use that one next time. There is only air in one or two places, and it's because the matting pieces bunched up with the resin and made little balls, which the next layer went over and made a small pocket.

I am using polyester resin, and I just bought another gallon and appropriate hardener. When I get done, it will be two gallons total. -- If I use all of this, which I prolly will.

My technique for fixing my mess is to go around with a knife and cut/break the little spidery chunks off of the box so that I have a smooth surface to work my next layer off of. As for cutting myself, that happens when I'm trying to clean off the pointy sharp little hairs that try to do me harm. Not too bad until cleaning your hands with acetone after. Thanks for the tip on the painting tech - I bought a 3 inch spreader/paddle hoo-hoo that seems to do the trick much better if I don't over work it.

As far as flexing, there isn't much now that I try again. if I grab the opposing corners and twist in the opposite direction there is a little bit of twist-action, but pretty small. I'm planning on using my other gallon of resin, for a total of 2 gallons. This should beef it up, with more matting I assume that will help too.

All in all, I'm slowly getting the hang of it, and when I get my ferrari I will be skilled/determined/diligent enough to make a supremely pimpin box. Heh heh, just as soon as I win the lottery I'll get one. Thanks for the tips, I'ma try to get some pics up before I ask how I should finish the top.

One question -- How does bondo and resin work with eachother? They are both polyester so I estimate that bondo will stick quite well to the sides of the box. I'm going to try and manufacturer a top this weekend, and I'll post back with pics.

same thing - if you get all the air out, and make sure there's something for the bondo to hang on to.. (reminds me - you should rough-up the surface of the resin before applying the bondo) .. oh, yeah.. should be fine.

fot a 3ft^3 box, how much of it is fibreglass, and how much MDF/etc...??

we want pics

I'm doing the same thing right now. I have used nearly 6 freakin litres of resin and I'm quickly going broke. Going to see a fibreglasser this week to try and get some resin on the cheap.

Would 3mm mdf work as a way to bulk up the walls?

How would you work this? Do you do a layer of matting then put the mdf down, soak it in resin then do another layer of mat?

Also how thick do i need the walls to be? I was lookin at CSA mag and their 'up front bass' box tutorial box seems to be ~20mm (had another look and it not that thick). I was working on the principal that if i normally would have made it out of 18mm (or 19mm i cant remember what it comes in) MDF that the fibreglass should be that thick as well. Am i correct in that assumption.

Sorry to hijack your thread Corksil but it seemed like we were both having the same problems.

You should pay about $14 per litre for resin.

To 'bulk up' the box, you would make sections of it from MDF and glass the inside and outside to bind the pieces together.

Mine is about 6 to 8 mm thick, about 7 or 8 layers of glass - seems pretty solid.

Hey I got some pics up finally. I posted them in my photo catalog thingee at this address. I will post a few of the more important ones below, feel free to explore the link if you hunger for more.

Here's a pic of the box from the top - we are nearing completion. As you can see, I laid trash bags down in the wheel well, painted on resin, and then used matting to make a rough base that would allow me to resin up over the wooden frame that resembles the top. After removing the box from the vehicle, I could pull off the bags, and gradually started building up layers and matting on the inside. When I got close to strong enough (I think) I sawed off the extra glass/resin over the wooden top, and screwed 2x4s onto the edge hanging in.


In this pic, you can see the bottom of the box. (I have plans to do a final coat of resin with finishing agent in it to smooth out some of the uglies and permit a light coat of paint. Not to mention a little extra strength...)


This one shows the little bunches of glass and resin that I was talking about. When you overwork an area, you pick up bits of glass which roll into little clumps like this - when the next layer goes over it makes an air pocket underneath. (Best way I know to explain what's happening.)


This pic shows the top edge that I cut down flush to the MDF with the jigsaw. I did the best job I could and ended up getting totally itchy and glassed... Atleast I wore earplugs, eye protection, and held my breath.


This is a shot of underneath the 2x4 after I screwed it into the lip of the box. I plan to use bondo to caulk/seal this crack and the wall into the roof of the box.


Few materials used in the process...


NOW -- after all that posting of pics, maybe I could get some feedback. Does my method sound about right, and do you think the bondoing of the 2x4 onto the rim of the box sound appropriate for strength etc etc.... I almost forgot - my plans to finish the box are such: after using up the last half gallon of resin, I will try to screw an appropriately shaped piece of plywood over the top of the 2x4s. I will use lots of glue/sealant/liquid nails in the process to seal the joint. I am going to look into mounting a 2x4 brace into the box vertically and if I don't do that, I will build the top of the box to be reinforced with 2x4s. I don't want my 50lb sub resting on the rim of the box and the top because I'm not sure just how strong a box has to be to withstand 50lbs dropping on it after a speedbump. (Not that I drive that fast anyways...) Once I cut the hole for the sub, I will reach into the box and seal the plywood/2x4 joint with my finger and some more sealant. Also, I need to add a port in the plywood top somehow. I will have to check the interior volume with peanuts again and work it out in WinISD.

How do my methods sound, and feedback? Thanks

I would glass up the inside edge between the 2x4 and the fibreglass rather than simply using bondo. Bondo won't give you any strength.

Also, you should really invest in either a respirator or even a simple dust mask at a pinch if you're grinding/cutting fibreglass. Getting itchy is nothing compared to what that stuff will do to your lungs.

There should be no reason why you are getting air bubbles in the fiberglass, make sure you use a cheap paint brush to apply the resin and tamp it down well, we use LOTS of resin and layer on the matting pretty thick when we make our fiberglass enclosures in the shop.

Make the box twice as strong as you think you need it to be

I would reccomend a minimum of 18mm as a top layer or even two layers of 18mm laminated with PVA glue if possible.

Use liquidnails to glue the 2x4 to the fibreglass section of the box, If you are keen you can use bondo or fiberglass on top of that too for extra strength. (Do it on the inside of the enclosure)

I glassed my 'top lip' onto the top edge - makes for an airtight, structural seal.

looks good... a bit rough, but good.

Just thought I'd mention that fiberglass cloth seems to be one of the easiest ways go get yourself out of the "oh snap, I am fudged" hole. It lays down very nicely, doesn't bunch, and adds a nice flat smooth surface to work from. Just somethin I've learned, it may be horrible technique. First time glasser

The crack around under the 2x4 lip is pretty small, and I have liquid nails'd it throughly - is liquid nails a good enough seal on a 1/8inch gap? Or should I try to reinforce it with something? I'm wonder how much pressure there will be in the box when the sub is hitting.

pressure in the box... I calculated approximately 200 kg/m/s for mine - more to do with ridgidity than actual pressure - the static pressure is irrelevant compared to the acceleration forces.

It looks like you have not used enough resin when applying the matting and because its not wet enough it has lifted up.

As long as the box is strong and sealed who cares what it looks like though?

edy

Excellent tip about the not-using-enough-resin. Now that I think about it, that is exactly what I prolly did wrong. Yes, my box will look like a fiberglassed turd, but hopefully it will work well enough.

Thanks for the numbers pulse-r, I read them as xxkilogram/meter^2 - correct me if I'm wrong. This comes to 200kg/m^2 = 0.2845psi or 40.96lb/ft^2... That's quite a bit of force.

hehe
no, it's kilogram per metre per second - an acceleration force - because the pressure is dynamic. it vibrates and resonates, so the layers can separate if it's not strong enough.
the actual peak pressure will only be about 1.5kPa, but at 60Hz, that becomes a lot more force. kPa = kg/m^2
+1.5kPa to -1.5kPa = 3kPa peak to peak.
3kPa x 60Hz = 180 kg per second per square metre (or about 200).