will try to speak for Ian - it's impossible to make him type... (for the ones who does not know who Ian Hardwick is - top fresco plasterer in US 25 years exp. Fresco School's plastering instructor for the last 8 years)
i assume that Durarock is similar to laymen's vocabuary (such as my) known "wonderboard" - that thing is a good for nothing when it comes to lime plasters (you should perhaps tell that manufacturer to staple his mouth shut about lime related advises)
Board has no tooth for the lime to grab (dolomitic lime might somewhat stick but somewhat is not good enough)
you will also have detachment problem if there is too much water in the mix especially if your scratch has cement (too much water will create a slick glaze-like seal)
(sorry for a "short-fuse" looking reply - i just cant stand when due to salesmans ignorance professional people like yourself get into the predicament)
here is the suggestion - you can find your way through it.
1. ideally you put expended metal and scratch over it (backing can be anything - drywall will do)
2 part lime - 1 part cement - 5-6 sand
when it sets hard float a heck out of it to remove the film if any.
2. Float to sand finish with 1 lime to 2 sand (sifted through window screen)
3. skim in two coats wet-in-wet. first coat same as #2 bring to a sand finish and the second one - skim (very thin) coat 1.5 - 2 lime to 2 marble dust.
you can perhaps combine 1. and 2.
you may find it very helpful to make a float like the one on this picture...
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