Gary, As mentioned above, I wouldn't coat the flat surfaces with anything. Normally you will surface your new board on both sides and I also used small amounts of glue to hold the MDF down to the vacuum board. I did however use wood glue on the edges which helps keep the suction coming from top to bottom instead of coming through the sides.
I will coat the sides and see if that will do the trick. My concern was if the mdf would curl and bend. I think I will have to try it and see.
I know I will be resurfacing as a newbie. The other problem is my table is too low for the router to reach properly. I was going to put a sheet of plywood for the base and was given 2 sheets of 1" thick mdf. This would get me back up to my height I think I need. That was why I was asking about both sides.