There are several rust removers available in Europe. You find them normally in drug stores, do it yourself warehouses or car supply houses. Just make sure they are based on phosphoric acid. One which is available in Belgium is called “HG roestoplosser” which means “HG rust remover” 
. It might be available in Spain under its Spanish name.
You can also buy rust remover from Cousins UK, which is made for horologists. Don’t know if it is different from general rust remover, other than the price 
. https://www.cousinsuk.com/catalog/consumables/watch-cleaning-rinsing-rust-removing-fluids/fluid-rust-removing-fluid#select
They ship worldwide and did ship rust remover to Belgium, so I suppose they will ship to Spain as well.
Jan