I've just started removing the under seal on the Cooper S (3/70). No horror stories yet except for the few rust holes I was aware of. it was a barn find sitting in it belly. I'm assuming they taped any holes for bungs/pipes etc before they put the under body seal on, so as to make the grommets fit better/then seal. Is this correct? Do I then replicate this procedure before I under seal/paint? Some were still intact, while others had peeled off around the holes. I've also included the obligatory fuel pump breather picture.

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1970 Mk2 Cooper S under restoration

IT is not necessary to tape the holes.
Undercoat the floor with a 2 pac primer.
Spray floor with stone guard available in spray can.
Spray all seams and joins with cavity wax,
available in spray can and long extension tube for hard to get areas.Rotate car if possible when using cavity wax.
I also have a CS 3/70 Jet Red which I did a ground up resto 5 years ago.
Restoring a car is not rocket science it is much Harder. Good luck Allen

Looking at some of your photos, I'd forgotten how much of the holes were taped from the underside.
If you were stripping the old sound deadener and the primer to bare metal from your car, I would just tape the inside of the holes - after all, a bit of sound deadener around the hole is not a problem when it isn't meant to be a waterproof grommet seal, just enough to protect the hoses and cables passing through.

When I began to strip the old primer and sound deadener on my rebuild, I realised there was no rust under them so decided to leave the original in place.

The biggest lesson I've learnt from my current restoration is to make sure you use a good metal primer under every other coating. My paint supplier didn't tell me that until I asked. Without the good metal primer, i was getting rust marks coming through the thin edges of the top coat.

Also make sure you know if your spray "sound deadener" or spray "stone guard" will accept a colour topcoat. On minis the underside is not topcoated but the wheel arches are colourcoated. Apologies if you know this already, but noone told me when I needed to know ...

Thanks for your reply Bill, much appreciated. I did some more this afternoon, a majority of the metal under the deadener is in very good condition - some not so. Because it was found in a barn on its "belly" some moisture had infiltrated under the under seal and loosened. I will strip the lot for peace of mind and to give it a fresh start - taking into account yours and Allen's recommendations on refinishing.

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1970 Mk2 Cooper S under restoration