You could lighten it up a bit with a "Dye" and cover with the HD40 not really the right product for that but I have seen it done. It's your garage do as you please just seems silly to go to all that trouble cleaning , HD40 and maybe a dye and still not be what you want. With 400 sq that is a pretty easy epoxy or poly job, though it is more money. I just know for myself if I do a "temporary fix" it becomes permanent, do it once do it right.
I used the Hd40 over bare and so far I'm happy with it. Hope it works for you.