You just have to keep at it with the coal slag.
It will eventually clean it up and using it on areas other than panels is not a problem IMO. I'm using it myself on the current project and it actually cut's better after a couple cycles as it breaks down and gets into a smaller area.
The media as it first comes out is rather large and most of it bounces off and WAY out into the area so a tarp is necessary to recycle it. After it breaks down some it will be easier to contain and recycle.Just be sure to clean the greasy gunk off first as well as the seam sealer cause it can get recycled into the next blast and gum up the metal with the residue causing you a world of trouble later down the road with the paint.The blaster nozzle is another concern as some have a very large opening and put out a lot of material which is fine for lots of area fast but deep rust needs a smaller nozzle to concentrate the media to exactly where you need it. If the pitting is "that" bad,the metal needs replacing anyway. Here is the firewall/cowl seam which is "toast" in most of the seam area but still some good metal left on the firewall. I'm having to "cut & paste" as no replacement panel is to be found which is a shame cause most all these 80's trucks are wasted here.
Here I have blasted the firewall and sanded the cowl. Most of this will get replaced with new metal but I wanted it clean to SEE how bad it really is,which is coming to "light" after the blasting.
After cutting out the bad metal I made patchs using a stretcher tool and round pipe to form the repop part along with a little work with the hammer.