I have just put together a sweet little workbench but my bench top is made of thick particle board or MDF. like what they make IKEA crap from. I want to seal it with a polyurethane or a lacquer or shellac so I wont gouge or cut the surface while I'm working on projects. I'm worried that they might start to break down the glue in the board or weaken it in some way. also what would be the most resistant. I don't want looks just durability. any thoughts?????????

About any finish is going to be 0-.006" thick. No finish is going to be resistant to cuts and gouges, hammer marks, etc. Once you get a cut in a film finish, it is going to fail sooner or later. I think you have several options:
1. Be really careful
2. Don't care if it gets beat up
3. Put on a sacrificial sheet of tempered Masonite and replace it when it looks ratty.
4. Make a slip on cover to protect it when doing "dangerous" work and remove it when you're doing fine hand work.
5. Melt 1 part of paraffin wax in 7 parts mineral oil. Smear it on and let it congeal. Scrape off the excess. This will keep glue from sticking. Refresh with more when needed.