This is an old expression from the art world. The technique of etching (=radering) goes all the way back to the Middle Ages. In order to obtain the finished prints from the etching, you need a press. As always in Danish, there are no clear cut rules about the spelling. The word can therefore be spelled in four different ways: rader-presse, radér-presse, raderpresse, radérpresse. I would personally prefer the latter, which accentuates the stress :-)