A few things stood out in your text that you may want to change, one grammatical, one stylistic, and one simply an omission.
"Painting these miniatures (are) not difficult, and a beginning painter should have no problems painting these miniatures." The "are" should be "is" as the actual subject of the sentence is "Painting."
"If you have them, you can (wash washes x) the "hole" of the pit toilet with Secret Weapon Miniatures Sewer Water and/or Baby Poop washes (as well as resin water or other gunk), to make it subtly more disgusting." Here you have what seems to be repetition though technically it's not. Stylistically it sounds better to replace the first "wash" with something like "apply" or "dump" since the very next word is "washes."
Then I think you left out the word "in" after "washes." Take a look and see what you think.
Going back to the idea of avoiding repetition, we could consider "painting" in the first sentence I copied. They're further apart so it is less noticeable, but you could change the second instance to "working with."
...and by the way, are you sure the wooden one is a chamber pot? I was thinking it was a bed for a baby with the lid for when it's crying.