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MG MGB Technical - Poly door lining

The MGB like many cars had a sheet of clear polythene taped onto the inside face of the door behind the door card. The polythene is normally taped all the way round to seal it.

Is the polythene there just to stop dampness coming into the cabin and damp damage to the door card or does it have other functions such as noise reduction?

Has anyone bothered to recreate it when putting the door back together after painting?

I am wondering whether to do this. I don't have any clear polythene but I do have left over blue damp proof sheet membrane from when I laid a concrete base for my new shed.
David Witham

i had it on my 1972 B,i think its there to keep moisture from getting into the interior of the car.All of new car today have a clear/white plastic lining. Hope this helps,Rich O
rich osterhout

The poly is there to stop damage to the door cards. Without it they suffer quite badly quite quickly. More water than you would exoect drains down inside the doors because of the poor sealing between door and glass. Check that door drain holes are kept clear. The blue sheet should be fine, fit it as accurately as you can and tape alround the edges. You can"t acheive perfection because the plastic pops which hold the cards on go thro holes in the inner skin.
jim soutar

When I replaced the door panels on my roadster 20 years ago, I used the poly covers , that the panels came wrapped in, to seal the doors. After all this time the panels still look like brand new. RAY
rjm RAY

I just used some clear polythene with afew pieces of masking tape to hold it in place until the door cards went on.

Water definitely does get into the doors as Jim says. I made sure mine have plenty of drain holes and they are drowned in rust proofing.
Simon Jansen

Also a good idea to paint the back side of the door cards - they are very absorbent.
Chris at Octarine Services

Chris, now you tell me! I just had both doors apart to replace the window outer seals. Drilled out and replaced 20 rivets. Now I know why i'd been putting it off for so long. RAY
rjm RAY

There is also a seperate, smaller piece of plastic that goes under the inside door handle mechanism before you bolt that to the door.
Simon Jansen

Simon, I have that bit. It was still on the car but a PO must have removed and not replaced the main sheet at some time in the past.
David Witham

For what its worth I took a line similar to that suggested by Chris and used a clear polyurethane wood varnish to seal the door cards. Cant comment on the results yet though since it was only done recently and I dont want to take them off again in a hurry - Ray i hear you!

P R Ives

Just dampness. My son had someone dent the door of his BMW, repaired, but they didn't replace the membrane. He parked it on the street at the time and the camber of the road meant that rain water coming through the window opening past the seals, dripped down onto the *inside* of the door carcase, and without the membrane ran into the car past the card. Eventually it filled up the footwell and I had to cut the foam off the bottom of the carpet, squeeze it out and dry it in the airing cupboard for a couple of days, as well as having a fan heater running inside the cabin. When I replaced the membrane I stuck it to the normal surface of the door at the top, but cut it to fit inside the inner skin apertures at the bottom so any further water coming in didn't lie in between the membrane and the door skin if it should happen again. Unless you car is regularly exposed to rain i.e. kept outside it shouldn't be a problem. I fitted membranes to my roadster when I restored it (even though it is mainly a fair-weather car) as it doesn't need much dampness to get into the cards to warp them, stuck on with double-sided sticky tape, 20 years ago. It's still stuck, and has restuck on the couple of occasions since I've needed to get inside the door, the last time just a couple of weeks ago.
PaulH Solihull

Well I hunted around the garage for some slightly thinner polythene without any luck so I used the DPM.

I hope the car is fit for our trip to Angouleme. We leave Thursday evening!
David Witham

This thread was discussed between 09/09/2010 and 13/09/2010

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