Collider for rocks.

I use something like this, its not a painter, and I'm not entirely sure how the painter works, but this will randomly populate a set area with rocks (drag the model including collider into the "item" field in the inspector and enter the other properties too to customise the item spawns

 using UnityEngine;
 using System.Collections;
 
 public class itemspawner : MonoBehaviour {
 
     // Use this for initialization
     public Transform item;
     public Vector2 spawnAreaBottomLeft = new Vector2(0,0);
     public Vector2 spawnArea = new Vector2(100,100);
     public int spawnQuantity = 10;
     public bool randomSize = false;
     public bool randomRotation = false;
     public bool randomRotationY = false;
     public float offsetY = 0f;
     //public bool dominant = false;
     Vector2 bLeft; //quicker! :D
     public float exclusionZone = 10f;
     //int i = 0;
     public string priorityTag; //when this item is spawned, delete it if it collides with the priorityTag item
 
 
 
 
     public void spawnItems()
     {
         bLeft = spawnAreaBottomLeft;
         for(int i = 0; i < spawnQuantity; i++)
         {
             Transform temp;
             Vector3 tempPos = new Vector3(Random.Range(bLeft.x,bLeft.x+spawnArea.x),0,Random.Range(bLeft.y,bLeft.y+spawnArea.y));
             temp = Instantiate(item,tempPos+new Vector3(0,Terrain.activeTerrain.SampleHeight(tempPos)+offsetY,0),Quaternion.identity) as Transform;
 
             if(randomSize)
             {
                 float _randomSize = (float)Random.Range(1,10)/10;
                 temp.transform.localScale = new Vector3(_randomSize,_randomSize,_randomSize);
             }
             if(randomRotationY)
             {
                 //Quaternion tempRot = new Quaternion(0f,Random.Range(0f,360f),0f,0f);
                 temp.transform.RotateAround(new Vector3(0,1,0),Random.Range(0,360));
             }
             if(randomRotation)
             {
                 temp.transform.localRotation = Random.rotation;
             }
 
             temp.parent = gameObject.transform;
             }
 
 
 
 
     }
     void Start () {
     
 
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 }
 

hope this helps