I got Animated Soundwave and he just looks fantastic, however, I'm not sure if there is a problem with the paint since the back is basically just melting away, I don't mind it that much since the color is dark enough that it isn't very noticeable, the problem is that for some reason the paint on his visor started to chip away, I don't even know why because this is the first part I in the transformation secuence and I always grab it by it's sides, not the front, and I want to fix it because it's driving me insane.
So, I was using acrylic paint mixed with some water and it worked at first but I messed it up and removed it, I was wondering if I could use a paint marker, but I heard that some of them have very poor quality, then I considered looking for a Gundam marker since it's also kinda glossy, but I heard that it breaks down ABS plastic, so I'm kinda on a dead end.
Should I try again with acrylic paint and a brush? If so, what technique can I learn to make it really smooth?

 

Not sure what you meant by this. The back of what is melting away?

Also, I always recommend Tamiya acrylics for Transformers. Any hobby brand should do, but my best results have been Tamiya, and it's even readily available and not too expensive. You'll have to thin it, as you did with the water in your first attempt. I've always used store-bought acrylic thinner rather than water, but acrylics are water based so water should work. I'm not certain what the best answer here is, but I would recommend trying acrylic thinner next. It's not cheap, but you only need a few drops at a time. One bottle should last a good long while.

Nice to see Animated Reprolabels still out there. I miss those sets.

 

I was talking about how the paint of the back is really messed up, almost like it's melting, it moves around whenever I grab it or touch it.

 

I've heard about that type of paint but never got to try it, guess I'm gonna be looking for it now, thanks!

 

What do you mean by „back“? Maybe it’s just the wrong word. Try to describe it or mark it in a picture.
If you paint something I highly suggest coating it.

 

I think the word I was trying to say was "backpack"
I don't know what it is about this part, it moves or slides everytime I grab it, maybe it's because my climate has a warmer weather?

 

I thought you were talking about the visor.
Yea, you should definitely let this completely dry first and then coat it.