Appreciate that it's a little late to be jumping on this topic, but I have also had a shockingly bad experience with thermally modified ash supplied by Carpentier (Belgium company).
I / (my company) placed a large cladding order for a luxury private dwelling in the UK (I was advised by the company that the order was the largest they had received from the UK at that time – I don’t know whether that was true or not, but it was a six figure order).
The sales process was very thorough, with lots of engagement and willingness to find solutions to fit the requirements of the scheme.
However, in short, within 7 months of the cladding being installed, the surface finish started to fail and flake/wash off. I raised this with the company and during the intervening months, in addition to the problems with the surface finish, it became apparent that the specification of the product supplied was different to that ordered and significantly, did not have the fire rating specified at the point of order.
Trying to resolve the issues with the cladding with the company was almost impossible. My experience was that they literally did everything possible to avoid taking responsibility or to resolve the problem. After 5 months of trying to resolve things directly with them and desperate to finish the project, we felt we had no option but to instruct solicitors (in Belgium). The whole experience was extremely difficult. To resolve the problem, it was necessary to sand back and refinish all of the cladding.
The foregoing feedback is based on my experience of dealing with this company. I cannot say, and am not claiming, that my experience is reflective of the company’s products or conduct more generally.