Prosthetic foam is a special material with extremely thin closed cells, waterproof. It has excellent elasticity, flexibility, mechanical resistance, it is a good insulator, resistant to low temperatures and UVA rays. Prosthetic foam is used in a large number of sectors, due to its versatility and resistance, from costume jewelery to solar panels. It is resistant to fungi and bacteria, hypoallergenic and non-toxic. It can be easily cut with a sharp knife or a long blade cutter in all the necessary shapes. Since the surface is similar to that of human skin, in lutherie it is mainly used to create pads of all the forms necessary for oil varnishing, facilitating pad-printing in areas difficult to reach with the fingers, such as the diggs of the scroll and the angle between the ribs and the planes.
You can read the complete William Scott article on how varnishing using prosthetic foam on "The Strad Secret 1".