RFID read --> String

Basically, I want to take each read byte from the RFID to add to a string as HEX characters rather than decimal, this is my code currently:

#include <SPI.h>
#include <RFID.h>

RFID rfid(10,5);

/*RFID VALUES*/
String readID;

void setup(){
  Serial.begin(9600);
  SPI.begin();
  rfid.init();

}

void loop(){
  /*Clearing*/
  readID = (" ");
  /*RFID READ*/
  if(rfid.isCard()){
    Serial.println(" ");
    Serial.println("Card is Avaliable");
    if(rfid.readCardSerial()){
      for(int i=0; i<5; i++){
        readID += rfid.serNum[i];
      }
      Serial.println(readID);
    }
  }
  rfid.halt();

}

This adds the decimal conversion of the rfid byte to the readID string, in the Serial port I get this:
Card is Avaliable
13414219150133

which are decimal, but I would like them in Hex for conversion.

Hi Jamackey,

Try:

Serial.println(readID, HEX);

Here's some additional information: Printing numbers in HEX with Arduino