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MG MGB Technical - spray foam insulation for gt

I am putting the interior of my mgbgt together and have cut some holes in the side floor panels to allow for the speakers. I am considering using some of that spray foam that is sold in the home stores that expands after you spray it on for insulation and protection of the speaker from leaks. Has anyone ever done this? It occurs to me that it should work pretty well but I'm a little concerned that the chemicals might somehow affect the metal. It sure wold fill all the spaces and probably provide some pretty good sound insulation. THanks.

Chris Kasten
Chris Kasten

Did on a Frogeye Sprite some years ago.....considered it when I rebuilt the MGB........you will find the thread in the archives......the majority were against the idea at the time, so I've pumped motor oil in all the cavities instead.

Mike
Michael barnfather

Chris,
Stay far, far away from expansion foam on your car. I promise that one day it will be hiding rust, and getting it out isn't nearly as straightforward as putting it in. My car's PO sprayed expansion foam into a few of the cavities, and I wish I'd been there so I could've stopped him. Preferably with a 2-pound sledge.
Adam Birnbaum

Chris,

First choice should be to purchase speakers with a rubberized coating that resist water damage. Cruthfield sell them. If you must use the foam make sure all is dry as the foam will trap moisture.

Regards,

L.C. '74 B/GT
Larry C.

Chris:
The thoughts on this foam are indeed mixed. If you look at the cans of foam in the store, before buying them, you will note that there are at least two types, vigorously expanding and not so vigorously expanding. The vigorous type usually cautions against using it in blind cavities where it can exert enough force to push things around, perhaps even sheet metal, and distort it. This type of stuff is usually used in applications where it can expand but vent its energy through some sort of escape passage. An example might be to seal between two panels like a gasket, or to fill a box or can with one side open. The excess flows out the opening and is trimmed back to suit. If trapped, it can buckle and bow things out of shape, or around house doors, can distort the door frame enough to cause the door to stick.

You can buy alternative low expansion type (as for sealing around doors) and that would probably resolve that issue. However, as the archives state, there is a more outstanding problem with some of these foams holding moisture, and hence, speeding the rusting process. It doesn't seem to matter whether the "holding agent" is foam or just accumulated dirt (as in many blind spaces on these cars), as long as it holds the moisture over time against the sheet metal, it seems to speed rust. Surface rust seems to grow more slowly when allowed to get wet and then dry shortly after. FWIW
Bob Muenchausen

Foam traps moisture which causes corrosion. Foam is nearly impossible to remove once it's hardened. You end up digging and scraping to get it out. Most are completely impervious to solvents, as well. Lot's of hard luck stories from restorers who bought cars that were filled with this stuff in the archives. Wasn't designed for automotive use. Stay away from it.
R. L Carleen

Thanks very much everyone, I'm going to give the expansion foam a miss. The last thing I want is something that will trap the moisture against the panels.

Chris
Chris Kasten

Chris,
I was just looking at a friend's Mini estate that has foam in the sills and behind the kick panels (factory sound deadening, it turns out). Yikes! It may have stopped some noise, but it sure trapped moisture and makes repairing the inevitable rust difficult.
David
David

Hi if problems with rusting potential aren't bad enough - The polyurethane foams are based on polyisocyanate and could give off dangerous fumes if burned. For example if one should one be unlucky enough to weld close to a filled cavity - or frustrated enough to try and torch the foam.
Vic
V Todman

I started off considering the foam route on my Frogeye, as at the time I owned an Alfasud.

They had a notorious reputation for rusting (probably because they were made from recycled baked bean tins and were shipped from Italy through the Med and Atlantic, and then parked in fields in the South of England awaiting delivery)

They had foam filled firewalls and 'A' pillars in an attempt to hold the rust at bay, and I assumed that this was a new ploy in the fight against the enemy.

I ended up filling the cills and front bulkheads with foam, when I sold the car it was still intact (it went to a Dutchman whose previous car ended up in a dyke, at least if he tried it with the Frogeye would float it had so much foam in it)

Mike
Michael barnfather

Vic,
Thanks for the heads up on the isocynate content of these foams! It is bad enough in paint, but why add to the problem with foams??
Bob Muenchausen

This thread was discussed between 18/08/2003 and 19/08/2003

MG MGB Technical index

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