This is what ive done.
Get a small bag of Quickcrete concrete mix from Lowes or Home Depot. They have small 5 or 10 lb bags so you dont have to buy a 80 lb bag. Mix a hand full in a mixing bowl with twice as much water then you would typically use for mixing concrete. It should be a soupy consistency when its right.
Take a 1 1/2 inch wide paint brush like you would use to cut in when painting a wall and dab the tip of it in the mixture. You want to stiffle the texture onto your rock. You want to just coat the entire rock with a skim coat. Wait for it to completely dry and reapply again waiting for it to dry. What you want to do is cover the entire rock to hide the pink color of the bondo and add texture at the same time.
Youll notice that when you touch the dry concrete on your rock it will crumble off. Dont worry at this point we will seal it later. Now take 2 spray bottles with water. Mix black Tempera paint in one and brown in the other. I start with the black first and mist it over the entire rock lightly. The dry concrete will suck the color right up so you may want to apply another coat when dry. Now use the brown the same way. Keep going back when dry and adding paint until your happy with the color.
Now mix elmers glue and water 50/50 in another spray bottle. Mist this over your rock, wait until it drys and add another coat, I like to put 4 coats on it. This will seal your texture and color and when dry for a day or two it will not come off. You can use compressed air to dust it off and it wont hurt the finish.
Here is a habitat I just did using this process
Author
