Theo,
Your English is just fine. It has to be...because I don't know any Dutch.

I think you are looking for products containing the chemical "Xylene."

'Xylene' is used as a paint thinner, and as a solvent. Hardware or paint stores
should be a source, as well as some fingernail polish removers. Read the label ingredients.

The chemicals 'Acetone' or 'Toluene' may also work, but from what I read,
'Xylene' is the best chemical to cut-through, or remove 'Balsam' based sealers.

My main solvent for removing glue from the few camera repairs I do is 'Lighter Fluid',
or Coleman camp stove, 'White Gas'. The main ingredient in cigarette Lighter Fluid
and Coleman camping fuel is 'Naptha,' (sometimes spelled Naphtha).
I do not read anywhere that 'Naptha' will cut or remove 'Balsam'.

Now that we have your needs translated, maybe others will be of more help.

Marc