Hi Barb,
I had to do this a few times because I really did a good job with the boil over. It may only need to be done once or twice though, Lol.
Use white vinegar. I put a little in a small measuring cup to control the amount more easily when pouring.
Dribble this in small amounts on top of the crusted area, covering it all.
Then shake out the baking soda just directly on top of the now wet burned material. It doesn't have to be super thick, just a nice solid covering. I played with amounts as I made the additional attempts.
It will immediately start to bubble when it comes in contact with the vinegar.
I tried 2 different heat approaches. I used an old hand towel that I folded in half. If it's just a very small area, you could use a washcloth.
I first tried lightly but completely wetting the towel, wringing it out well and then folding and heating it in my microwave JUST probably FOR A MINUTE or less IF you feel comfortable doing this...(I used to heat towels for my patients this way to use for a heating pad back in the old days at the hospital.) But you have to be careful not to over heat or burn yourself because the towel can become boiling hot very quickly which becomes evident when opening it back up. Then place it over the area.
I then tried another way of applying heat that felt like it might work better......I boiled some water in my electric kettle then placed the folded towel on top of the baking soda area. Then slowly poured the hot water directly on top of the towel, enough to saturate it.
Either way, leave this in place for about 15-20 minutes to penetrate.
Then wipe off and rub some if necessary.
Repeat as needed until done.
There are no scratches that I can see from using these products.
I tend to overexplain sometimes
I hope it makes sense.