I have an 18x36 gunite pool that need a 'renovation' - it hasn't been opened for many years. I am being told by a pool guy that I should clean and marble dust the pool. Is it common to 're-gunite' the pool first, then marble dust it? Is marble dust done instead of a 're-gunite? How often does a pool need to be 're-gunited'? Any guess as to the cost for 1-'re-gunite' and 2- marble dust? Thanks in advance, guys!