take a quick look over this thread:
Thinset Mortar for fake rocks?
also click on my favorite link in my signature. V V V DOWN THERE V V V
there is also this link for ideas:
Lizard care, and fake rock wall landscapes
your best bet considering your application is probably something waterproof. a waterproof grout is easy enough to find. shouldn't be an issue getting one at just about any local hardware/building store. not everyone carries everything so call around first to save yourself some time.
the reason i say "your application... and... waterproof" is because you mention java moss. there is 3 things necessary for it to grow but it looks absolutely astounding in the right application. ferts (most, if not all of which will be provided by your water source and animals residing within) are important. light is important, but easy with the right commonplace knowledge. it's all about the right spectrum for your application. after that you figure out your intensity. most importantly is water. java moss cannot be dry or it dies. it must stay constantly wet therefore it should be put underwater, beneath a nozzle on a dripwall, at the top of a waterfall, bottom of a streambed, ... you get the point.
vinegar should only be necessary for products including lye. ie. mortar or concrete substances. it is not a bad precautionary measure in the case you don't have means to test your ph. if you do test some tap water then pour it in your viv. test the ph of the water in the viv in the morning or after mixing it up pretty well. dropping your pump in the viv for about 5 minutes should mix it more than enough to give you good results.
for your great stuff all you need to do is scuff it. no matter what rock like product you use (mortar, grout,...) the first layer probably won't cover all the way. if you check out that video in the other thread closely you might notice when he shows the closeup you can see some pink vaguely. as you apply subsequent layers the bond will cover more area better. usually everything is covered quite well on the second coat. each coat makes the structure stronger with the cost of losing fine details in your original structure so thinner mixes are better.
this is why it's better to use a powder because some mixes are not exactly water soluble when they're premixed and if they are you're paying for added weight and water at every step along the way. like shipping, packaging, manufacturing... so that guarantees that not only do you get a product you can work with but you don't pay extra to ship water from one distribution center to the next. it's a win win imo.
btw, here's a quick update on that crabitat... now that i know how i want it i'll be cleaning, stapling and siliconing and i'm done... yay.
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