Fiberglassing a horizontal surface

Hi.  I searched for the answer to my question and was unable to find it...  My car is a 1990 Mercedes 300ce (coupe version of the e-class).  For those unfamiliar with the early-90's E class cars, the trunk is sealed off from the cabin because the gas tank is mounted vertically just behind the rear seats.  Since the trunk is so sealed up, high fidelity bass cannot be had without some modifications.

My subwoofer installation requires me to cut a hole in the rear dash and mount the sub inside the cabin firing straight up (just like the rear speakers).  I want to use a sealed enclosure.  The underside of the rear dash (i.e. the metal surface inside the trunk) is not exactly flat--there are grooves and various other angles to it.  I would like to use fiberglass since the enclosure will be suspended by the rear dash (i.e. "hanging" from it).  So MDF is too heavy.  I also want to use fiberglass because it will allow me to mold into the non-flat underside of the rear dash.

So, now that you all have an understanding of what I'm trying to do: how do I go about fiberglassing a horizontal surface?  I have read about people having trouble because they can't get the FG matting material to stick to the first layer of resin.  I can see how this could be a problem for me, especially since the area is probably 3.5' x 2'.  Would I be able to pop the mold out only after putting down a single layer of resin (i.e. no fiberglass)?

I've never used fiberglass before, so I'm not sure how tacky/sticky the resin mixture gets.  Thanks very much for your help.

That's great advice.  Thank you very much!  It sounds like this is quite a production, but I think it will be worth it.

Follow-up question- I know the normal process is to apply a coat of resin first, let dry for a while, apply one layer of FG, wait, apply more FG, etc etc.  If I apply 3 coats at the same time to the tinfoil/Foam/MDF apparatus and compress it against the underside of the rear dash, how will this affect drying time?  Could i maybe just do one layer of FG first, let dry, remove the mold, and then apply more FG while at my workbench?

Thanks for your excellent advice!

i have no idea what that other guy is talking about..with the pam and aluminum foil..but anyway...

first the foil will release from the fiberglass without the help of cooking spray..

second...fiberglassing vertically and on undersides is not impossible...it is just very difficult as the mat tends to sag...and resin drips everywhere..

best thing to do is mix your resin a little hot (by a little hot means to add a couple extra drops of hardener to the resin so it cures faster) and then apply a layer of mat..besure the peices are torn and not cut...as the frayed edges will tend to stick better then cut ones...plus the seams are less of a pian in the butt to sand...just be sure to tape and foil over everything and anything that has a chance of getting resin on it..including the route you personally will be taking to get into and out of the car...a drop sheet will work fine too...

also using a roller to squeeze out excess resin will drastically cut down on the amount of sag...if you are going to spray pam on foil i would only do it to the piece on the application side IF your going to go that routeand then roll the foil on with a roller to again squeeze out the excess resin..

only do one layer of mat at a time on vertical and upside down surfaces, to reduce wieght of the first layer...subsuquent layers will not have as much trouble sticking as the first...

regarless of the finished shape of the box you should leave it in the car as long as possible,  as fiberglass will warp and twist once removed from the forms holding it...(you car) i would not recomend the sandwich technique either.as you will only be able to do small sections at once.

like every other install if you have large flat sections you should build them out of MDF..as weight should not be your only concern..flexing sides of a box will damage your subs and sound like poop...

So you're saying that the FG won't fall off/sag? I'm considering another approach to the project- I think I might just use MDF and use some thick foam material between the underside of the rear deck and the MDF.  That will keep it from rattling.  Then I can just avoid the whole FG mess...

Thanks for your help guys.

The dry time will be longer but in the end it will be faster to do it 3 or more layer's at a time and this will waste less resin as well. Doing it one layer at a time is time consuming as well as you waste more resin this way.

The "other guy" has been doing this type of fiberglass work for the past 8 years and have built many systems with different techniques. I told the member Pam because it's an oil based biodegradable spray agent that you can buy anywhere. I could have steered him to PVA  but unless you know what it is or know whereto readily buy this, it's not worth mentioning. Heck, even silicone spay or Turtle Wax would work too as long as it's a barrier between the fiberglass and the tin foil.

With a release agent on there it will just come off. Without it, you will need to peel it off with your finger nails and if there are wrinkles in the foil, good luck trying to take the foil off the mold.

This is why I suggested applying the mat & resin onto a peice of foil first & then taking this foil and appying it to a masked off area with the PAM. Using the foam & MDF board will allow the pressure to be applied to areas that have dips and rises therefore allowing you to be able to do more than one layer at a time. With the added support of the MDF, there will be no sagging and the foam will take care of the dips in the underdash area where it would normally just fall. Using something to hold up the pressure for the MDF & foam will give the fiberglass a  smooth contour to the base of the underdash.

i meant nothing bad by it man...i just didnt understand what you wrote ( how it was written)...i agreed with the pam..and if your gonna sandwich build it fine...but like i said that is only good for one panel at a time...

by the other guy i just couldnt remember who wrote it and the little window at the bottom of the screen wasnt open and i had to open a reply window..therofr not being able to scroll up and see who said it...sorry

i only said do the first layer slow...after it has cured you can definately do three at a time..plus using a roller will get rid of excess resin..

also how do you sandwich a panel that is all full of dips and crap that most folded steel and plastic panels in a car are? i have done it on flat panels before but not on multi-dimentional surfaces..