I have had maple countertops in my kitchen for over 30 years. I have had very good luck with a conversion varnish from Sherwin Williams. It's called Sherwood and is a 2 part that must be sprayed. Dries FAST and hard enough to sand is an hour or so. It's pricey, but it works well. I did it last time about 15 years ago and it's about due for another round. As you might expect, the areas around the sink and the cooktop take a lot of abuse, but nothing is peeling or coming loose. I love maple tops, but I only have a couple of small ones in my shop. I just can't bring myself to beat on them so a lot of utility is gone to avoid banging them up. If you're going to do any serious work and cover them up with masonite, the "niceness" of the top is kinda lost and you might as well us a solid core door or something more practical. For my next top I think I'm gonna use a piece of wide LVL. Planed down and covered with Masonite or something expendable and easily replaced.