Grout: leaching or sealing

[Tarkah]

Trying to research different methods and stuff for my project. Which I shall be posting in the construction section soon

But for today I'm curious about this grout sealing business.

So far all i've come by are grouting styrofoam and then sealing it. But ive come to realize that most of these were for desert typed environments.

I'm planning on making a rock wall for a CWD enclosure.

so whats the difference between leaching and sealing? Does it matter or is it just personal choice?

I've always been told that grout is only semi-waterproof and water still soaks in, hence why we need grout sealer partly.

So how is leaching it going to make it waterproof?

Secondly is grout really needed?

My viv is going to be 4' x 2' x 5' So grouting or GSing the whole thing is daunting and a little too pricey with a viv so big.

So the idea was to create slabs of styrofoam to create a rocklike feel.

So do I really need to grout the styrofoam blocks? I know that some acrylic melts styrofoam, but is that the only reason other than realistic looks?


Another option i was wondering is have the styrofoam in the right shapes and carved, then using the greatstuff instead of to carve and shape as a sealer like some do for wood.

So liek the greatstuff when it cures becomes waterproof cause of the skin.
So when its still wet and we spread it to thin it out, when it cures it will still be waterproof as long as we don't carve into it right?

sorry for such the long post!