I'm by no means a weathering expert, but I have looked at a lot of rust in my time. So here are a few thoughts that came to mind:

1, I would probably stick to finer grains for your rust. The larger sand grains seem to be a little on the large side for 1:25. Maybe if you look at the model in person it's not so obvious, but when you look at magnified photos, like if you were standing next to a real car, the 'rust' bits are quite large.

2. I'm not sure of the purpose of painting over the sand. If you were trying to imitate rust bubbles, they usually start from a place where the paint coating has been compromised, and the rust 'grows' under the paint, separating it from the steel below, creating a bubbled appearance. Typically this would start in small areas (like a paint chip, or a panel edge) and grow to the point that a large piece of the paint would flake off. You wouldn't typically get large bits of rust with little bits of paint on them, at least on the cars I've seen. Also, when the rust bubbles become quite large, it's not uncommon for a hole to have been created under the paint, that suddenly appears (much to your chagrin) when the rusty paint flake is removed. I've also seen rust bubbles occur when the panel has been rusted through from the back side, which immediately would reveal a hole once the paint is poked at (have even seen water drops emerge from those rust bubbles when poked).

3. The location of your rust areas on the body look to me like perhaps areas where paint was sanded down and then left out in the elements, where they started to rust again. If that's what you are trying to replicate, then it looks right (except for the paint on top of the 'rust'). Otherwise, you should sort of come up with a theme, for example, that the car sat outside in a desert junkyard for years and the paint on the top surfaces was baked by the sun, and rust started there. Or, it was exposed to humid conditions or salt roads and rust started from vulnerable areas, or damaged areas, etc. IMHO, you should pick a theme, then search for photos online to see what it should look like, and try to match that.

4. Another thought would be to reconsider what you are using to represent rust altogether. Years ago I attempted to make an undercarriage look like a car that had sat in a damp junk yard for years before being dragged out and turned into a stock car. IMHO, looking back I think I overdid it somewhat, but the material I used was real rust - I took some steel wool, and placed it into a jar with some water in it (you could make a salt solution to speed up the process). I let it sit in the open jar for days or weeks, letting the steel wool rust. Eventually the water dried up, leaving behind rust powder that I could further manipulate into a fine powder. I also found some rusty steel car parts that I had left out behind my back step (just by happenstance) and sanded off some of the rust from those parts, collecting the powder into a baggie. I painted the undercarriage flat black, and then sprinkled the rust powder onto the wet paint, brushing off the excess after the paint dried. The pic below illustrates that I was a little heavy handed IMHO, and if I do another one I will apply the rust more judiciously the next time around, so the rust is accentuated, not just one big rusty blob, as this appears.

I don't consider the photo below as a tutorial on how to do it, but just an example that the tone/finish of real rust is perhaps the most realistic way of achieving the look. However, practice makes perfect, and to be honest, I never perfected the technique.

Keep up the good work! You are definitely headed in the right direction!