If you're doing a frame off restoration and taking your Isetta all the way back to bare metal, probably one of the more dreaded task is removing the Isettas sound deadening material. The original sound deadening material is a tarred felt that was glued to the body. If you're a real purist and want to use the same material, a virtually identical sound deadening material is available from LeBarron Bonney. Another item that LeBarron Bonney sells that is very similar to an Isetta material that is used between the frame and body is an adhesive backed frame webbing.
Since the sound deadening material is something that is not seen once the car is assembled, I chose to go with a better performing newer material. I used a product called Hush Mat. I chose to go with their regular black polymer psa dampening pads but they also make Ultra Hush Mat which is foil covered. It took me about 48 square feet to do the interior of my Isetta.
Hush mat cuts easily with a razor but the pressure sensitive adhesive can be a little bit of a sticky mess. I found it helps to do the interior in smaller manageable pieces rather than larger pieces. The larger the piece, the more problems I had with air bubbles and correct positioning. They sell rollers on Hush Mats website, but I found a roller not to be very effective. My hand and fingers worked much better at smoothing the Hush Mat down over the beading and contours of the body. One item I did find helpful was a set of punches for punching holes in the Hush Mat around bolt heads.
An additional fiberglass matting was used on top of the tarred felt material in the firewall, parcel shelf, and behind the side interior panels. The front door used the fiberglass mat but did not use the tarred felt material. I've been looking at a few different materials to replace the fiberglass matting with, and will update this post when I've made a decision. Looking at this photo also reminds me of something else I discovered awhile back. 145R10 radial tires don't fit in the spare tire space of the Isetta very well. The original bias tire fits much better.