Which materials are safe for building my new cage?

[CakeLike]

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.

[WhompingWillow]

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.