It looks like youve already removed the veneer. 80g is way to much for veneer, esp mahogany. Youre looking at glue staining below veneer I think. Hope Im wrong.

This test board had a blotch too. Forgot where.

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I am going for this look. I found it on Houzz.

This is what it looks like now...

I'll try to back off to 100 grit tomorrow. I do a quick pass with 80. I'll have to sand longer with 100. I do the least passes I can with 150 and 220. Paying attention to not pause anywhere too long.

I should say I don't think the veneer is sanded off because as soon as I hit one of those areas it doesn't lighten up like the rest. And I don't try to keep going at it because it won't really help. I am doing a bigger test board behind the washing machine. It is one of the boards I showed early on. Perhaps when I stain it there will be no issue (that would be nice) Was just kind of wanting to finish sanding but now I am a bit sheepish. Suppose I need to stop and test to see how it will work out with stain on it? I am thinking of doing a dye and mahogany filler mixed with some stain. The current stain has a dull grey hue and it is dinged up over the last 60 plus years. I want a tiny bit redder brighter like the photo. And not so dull.

I opened a bigger scale of the photo of my current laundry above and it is a bit darker around the outlet. I just sanded it and that is the spot were it is darker. Never really noticed it on the original.

It is always in the middle of a panel as if the person who coated it soaked their brush or rag and then went for the middle and then rubbed it out to the rest of the board. I think it is definitely the same muted brown with a touch of grey stain used in my living room on knotty pine.

Mahogany that Im familiar with, is dark almost like walnut. Im not seeing that in your top pic. Looks like it could be the glue surface. I really have no idea what you have going on but veneer can be so thin that one heavy pass of 80g with a good sander can vaporize it. Usually no stain is needed to get a dark finish with mahogany. Just very light sand (220 by hand) and varnish will be your best bet.

My mahogany looks and receives products just like the ones in this tutorial.

https://www.youtube.com/watch?v=Rg7tFu1NPjg

Miine is ribboned grain. There are many varieties of mahogany. But yes when I put any top coat on the test piece it turns dark as if it were a stain.