Air...
Great first! Yes, like others have said, it's a bit out of scale, but you have the basics down!
I am a fan of spackle DIY zimm. Actually, I use pre-mixed dry-wall compound and mix this with sand-colored latex wall paint... the paint adds an appropriate color and makes the product a bit tougher and more adherent when it dries.
As I see it, "compound w latex paint" offers several great properties and some advantages over other DIY materials:
1) Achieves the correct (and with drywall compound) fine-enough cement texture to effectively emulate the cement-like surface of real zimmerit, and it's probably pretty scale, too! BTW- you cannot get this surface using any of the plastic or epoxy putties and it does not happen on either styrene or resin zimm- these things are too smooth, IMO. But this may not be much of an issue to everybody!
Notably, if you thin the spackle with paint, you can actually apply a thin-enough layer to be in scale (real zimm coats were less than 1 inch thick - so for us that means only maybe 1/35 of an inch thick (about 0.03 inch or less than 0.8mm) - which is pretty thin.) - this layer-thickness is crucial to getting the sculpted zimm to look in scale... think "brushing on and smoothing with a card or knife".
2) Can be easily and very realistically chipped and spalled- compound behaves just like miniature cement, so works well for damaged zimm.
3) It does not damage the kit nor scar it in any way, so can be safely removed...
4) Following on 3, compound is easy to wash off (before it dries) and so mistakes can be fixed easily.
5) It's CHEAP, so you can do lots of it and try stuff.
6) It paints well.
7) It's safe to work with - probably about as safe or safer as any modeling stuff we use!
8) Any sort of tools will do!
While I do like the look of some of those other products- such as current molded-on zimm on some Dragon kits, and the ATAK resin sheet stuff, DIY is ultimately a better thing IMO, as you can indeed create whatever damage effects you want and the finish cannot be better! ONLY by doing the DIY can you actually get the correct "cut-under" seen on zimm.
Of course, the down-side is it can be tedious to do the sculpting...!
IF I knew how to post a pic HERE, I would - But you do have the idea! Just keep practicing!
Just my opinion, gents!
Bob