Which materials are safe for building my new cage?

I am in the process of making my snoot a new cage and I build the background of the cage according to tutorials I found on the web.
I used landscaping foam declared as fish safe.
But now I want to paint the background and I found out that many people use grout to coat foam backgrounds but I am worried that this could cause injury in my snake since the material is rather rough and he has gotten himself scratched in the past from not so rough surfaces.
I have heard this shouldn't be a problem but still wanted to ask in case.
The other method I have found on the internet is to coat the foam in silicone and put eco earth on top of it. This would be the other option. I would like a stone wall look though so grey coloration would be ideal.
But if it can't be done safely I will opt for the safer method.
Here's a picture of the build:

https://www.instagram.com/p/CY3m9kHM...dium=copy_link

If you have any other ideas how to achieve this and what materials are safe (kinds of paints and such) please tell me as well. Thanks.

We used Original Drylok in white to paint the inside of our Bredli enclosure. It's a bit rough but has the advantage of helping to waterproof the cage. They make it in gray, too, and it's pretty easy to work with.