ECKey.ECDSASignature creation
Signing the bytes of the private key of ECKey key is unacceptable because you are vulnerable for realeasing information about your private key.

In the case of signing a message(as a String) you have to estimate the :

 byte[] result= sha256(message)


The result will be a byte[] and this array finally will insert to the :

 Sha256Hash input= new Sha256Hash(result);


Then the procedure is simple in order to produce the ECDSASignature.
You have to do the following:

ECDSASignature signature= key.sign(input);


and finally in order to validate your signature you have to check the following:

key.verify(input, signature)