Unhaired lime mortar and plaster over diathonite..?

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Hi folks,
Although I joined up last summer, I think this is probably the first time I've posted, so I just introduced myself...

As I said, we've been re-rendering the internal walls of an old Mill House we're re-building. We've used diathonite on one of the walls, which is nicely curved, so diathonite seemed to have advantages over the multipor boards we're using on other external walls. I'm posting this question on behalf of our plasterer who isn't a member here (and whose computer is broken, he tells me..!).

He hadn't used diathonite before and although we were assured it could be applied by hand, it didn't go well, constantly sagging off the wall and in the end we both gave up. I got someone in who is very experienced with diathonite and has a plastering machine but unfortuanately that didn't go well either and it left us with a wall which was basically quite a mess - a huge contrast to the lime mortared walls which our plasterer has done a cracking job on. He's now applied several small amounts of diathonite and has mostly rescued the wall but we're now running low on diathonite and there are some areas where we could do with adding a far amounbt of material to bring curves out to look right and to finish window and door reveals.

There's enough diathonite on the walls to insulate them fairly well, though less than we intended, so we're both wondering whether it would be possible to bring out the walls and shape them with (unhaired) lime mortar - say 10 to 20mm in places - where we need to, over the diathonite, before finishing with lime plaster? If it's possible, would he need to put a harling coat on top of the diathonite first?

He's also a bit doubtful about the advice I've been given re lime plastering on top of the diathonite - a 5mm coat of lime plaster has been suggested, which is a lot more than he would usually use on top of the lime mortar. Any views re that would be welcome too.

Many thanks, in anticipation,
Thanks, Malc... yeh, I thought of phoning them / Ryan - it's where I bought the diathonite... I'd really like to hear from plasterers who are using the stuff regularly if possible.
I can't readily see any reason why we shouldn't be able to put lime mortar on top - it would certainly give a better finish for lime plastering, the diathonite is much rougher than the unhaired lime mortar, with lots of 'pits'. Our plasterer would normally only apply a top coat lime plaster of about 1mm over the mortar - he was a bit horrified at the 5mm suggestion.
Quick update for anyone to refer to in future:
After talking with Ryan yesterday, I decided to call Diasen, manufacturers of diathonite, in Italy.
Their response was that applying lime mortar over diathonite would not be a problem.
I think they wondered why I'd want to, though, and were a bit concerned that I'd had problems with their product, so their UK rep should be contacting me to come see the house and advise. I may update again after his visit.
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