Preparation of existing marblesheen finish for renovation

[oscarcat]

Hi,
My pool is 20 years old. The marblesheen is stained and in one or two places small patches of concrete are visible. Otherwise the surface looks fine; there are no visible cracks or holes, just stains from the copper basded algicides used over the years. We want to renovate and change frommarblesheen to fibreglass.
Two completely conradictory methods are being proposed (from reputable professional pool renovators). One involves the complete removal of the entire marblesheen finish back to the concrete base which it is claimed is essential; the other merely grinding off the loose bits, rendering the patches, acid washing the remainder and then laying on the new fibreglass finish.
Each camp ridicules the other! The one group claim that the entire existing marblesheen surface must be removed at considerable cost); the other says that this is just not necessary and that an acid washed surface is perfectly sound on which to lay a new fibreglass surface.
Which option would members choose?