I was reasonably happy using zinc plates for etching but I have found that this metal has an adverse effect on colours of inks used on plates to print. I have been informed since that this is particularly true of colours containing yellow as this colour is particularly affected by the zinc. This can be very disappointing to find that the bright colour you have mixed has significantly changed/dulled.
When I found out that copper plates do not affect the colours I then believed that they were worth the investment. However I found that a zinc plate that perhaps only takes five minutes to etch can extend to the region of one and a half hours for example with copper. It is a tedious job to monitor closely the etching process developments with trying to bite copper. Even then the length of time spent in the acid can easily effect the soft ground you use and spoil the results I have found.
Even with etching line with a hard ground base can prove very unsatisfactory I found after the acid strength needed to be increased because waiting for results had become so tedious. The final increase in acid strength even spoilt the results again because it affected the stability of the ground. In places small spots covered in stop out apparently had lifted causing significant flawing and surfaces had many tiny spots of damage which would barely polish out.
It seems that etching with copper in comparison with zinc is an entirely different ball game. Perhaps experimenting first with small pieces of metal is a less costly mistake.
With two out of three tried plates unsatisfactory, I decided to return to using zinc, because I could not bear the disappointment and frustration in addition to the waste of cost.