My German is rudimentary, I take it that is regular fabric softener. I have used it before I got some Supersoft II. The fabric softener does work for bending if mixed about half and half water, a little improvement bending. The SSII is a little better. It will not turn the wood into rubber but it does help enough on problem woods. For the guitar in question I would wet the side and using a caul inside, use a clothes iron on the outside and steam it flat. I have done a side that cupped like that but before the linings were glued on.
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Fred
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