There are probably a number of things you can do to protect it from moisture, but a couple coats of polyurethane will probably be sufficient. I would love to build a traditional bench but cost is the major factor here. I find it can be really hard on saw blades. Dead flat, no fuzzy scratches, and I can clean it with soap and water and put a layer of wax on it to make stuff slide across it. As for edging, you can glue on a veneer strip or wood trim. My relies on decent vertical alignment as well.
©2019 Hearst Magazine Media, Inc. . If it is realatively smooth to start with, then a quick going over with 240 should be fine. What I have done is use my scrap sheet goods to make 2 lower layers. I pieced together the cutoffs from the outer layers for the middle layer and glued the whole mess together with titebond.
These guidelines, and more, can be before posting. We get a lot of humidity here and my shop is in my garage. When I lived up north, all my shops were in the basement and didn't have the moisture issue. You can mitre the corners if you want, but a butt joint will do just as well. I find it can be really hard on saw blades. I haven't wrapped the edges in hard wood yet, or waxed the top.
The glue-up you are suggesting, while possible, will probably be stable in the dimension of the horizontal plane, but it will be prone to warping and cupping. It is also in the middle of the room. Brush on and let this soak in and recoat if you want. This is not a place to advertise products and or services. Door had a tiny scratch so was marked down. Thanks for contributing an answer to Woodworking Stack Exchange! I've made some neat hardwood boxes walnut, rosewood, maple, etc for my dado set, chisels, and a sandpaper storage box but they don't get used as often or as hard as my bench.
Some Masonite comes with a sealed side and that works well too. The bench will be better for it. Re-treat whenever the surface begins to get grungy. Did he just say old growth, quarter sawn mdf? Then feed these shortened sections through the planer to reduce their thickness. After clamping, use a cabinet -scraper to remove any hardened globs of glue. All, I'm building a modest cost workbench for my shop which will use two layers of 0. Just to make sure I got anything oily off.
And the poly top makes it water proof and durable. Or the bench will be not against a wall. Also, I wouldn't go through the trouble of sealing mdf for a bench top, just my opinion. The bottom side is raw but has a drywall screw in it about every 3 inches in each direction attaching a web frame to it. Here is a photo of one of my benches going together.
In the end what ever you put on your bench to finish it is not near as important as what ever you finish on your bench to take off. Wood magazine has an excellent article about the different workbench tops - everything from the traditional butcher block to a solid core door. So unless yours is significantly larger, I guess I can expect sag. I'm thinking that if I go with the Hardboard top at least I can remove it and replace it should it gets too beat up. The second coat will not soak in as much so wipe off any excess after letting it set for 15 minutes.
The hardwood will move differently. The sawdust is very noxious and older engineered wood contains formaldehyde. I do not glue down the top layer and attach from the bottom with screws. How would the connection process go to attach the layers to the bench and each other? The I use spray contact cement on one surface to piece together. Although the plans are simple it is a very sturdy bench. Any that goes over the side will start dripping. The Murphy Bed is awesome and albeit has quite a few imperfections.