I personally wouldnt use silicone as a sealant as it can degrade. i would ise polyeurethane or some other liquid sealer. here are a couple links that might help: reptile terrarium rock wall background Gather Materials and Tools. This one has more detailed instructions, it is actually for a cave but same principles apply to making a background. Look at this link first as it will probably help you the most. Building a Custom Rock Background | The Reptile Report Fake Rock Instructional for a Reptile Enclosure Another fairly simple step by step How To Make A Realistic 3D Background For Your Aquarium Or Reptile Tank Not quite as helpful as the others but still an ok read. It does have a slightly more complicated way of doing it.