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);

String readID;

void setup(){


void loop(){
  readID = (" ");
    Serial.println(" ");
    Serial.println("Card is Avaliable");
      for(int i=0; i<5; i++){
        readID += rfid.serNum[i];


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

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

Hi Jamackey,


Serial.println(readID, HEX);

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