What are the pros and cons of MDF as a workbench top?
Pros:
- Relatively cheap.
- Is already a flat sheet ready to use as a work surface, saving much time and effort.
- Fairly flat to dead flat right from the start, but, needs good support to stay that way — MDF, like chipboard/particleboard, can sag under its own weight if only supported near the ends.
- Although the material is sort of friable, it's strong and cohesive enough for use.
- Uniform texture throughout, so no knots or other hard spots.
Cons:
- Not as strong or as stiff as solid wood (even pine and other softwoods are stronger in certain ways).
- Doesn't tend to take screws or nails as well as solid wood.
- Possibly outgasses low levels of formaldehyde1.
- Most MDF doesn't take being wet with water well and can swell badly. But moisture-resistant versions are available — called MR-MDF in some markets — which in addition to the moisture resistance are also generally tougher and stiffer.
- May not be able to be resurfaced properly. By comparison scraping, sanding and even planing the surface of wooden benches to refresh them is commonplace, as and when needed2.
I'm a newbie, so I was thinking that I could start with the 2 x 10s as a base, perhaps rip them so they can go over a friend's jointer and be mostly flat (they are currently cupped, up to 1/8").
Yes, reduce the wood to smaller, workable pieces as needed to process it into your workbench's understructure.
Since you're a newbie, you'll notice that once cut down the wood will exhibit less cupping3.
Glue them up and get them reasonably flat, then put the MDF on top to make it sturdier and flatter.
There is a major drawback to this plan. MDF is dimensionally stable but solid wood wants to change width in response to differences in temp and humidity through the year. The MDF will essentially stay the same size. While you can accommodate for this sort of thing in certain ways it's generally a bad idea to try to directly attach board materials to solid wood once you go past a certain width, perhaps 18-20" you're into an area where the side-to-side dimensional changes of the solid wood are significant enough that you need to pay attention to them.
What to do instead
However you don't need to ditch your workbench dream using this material entirely, quite the opposite.
Using what you have at hand you can make any number of different workbenches of various styles and configurations. At the most basic a single thickness of MDF can work as a top for now, with the plan being to beef it up later, and possibly add a sacrificial hardboard surface as well. If you have enough material you can laminate it now — a full sheet is 8'x4' and ripping this in half yields 2' pieces, and 24" or slightly less is a very common workbench depth.
There are various ways you can use the fir.
~2 inch material can be used directly and make for a very strong, stable bench. Material of this thickness has been used to make many a woodworking bench that has served its builder well, and outlasted them, as long as the rest of the construction was sound.
A more modern take is to rip the 2x10s into narrower strips (into 2, 3 or 4, depending on how thick you want the final top to end up), then face-glue them together to make a top where the board edges become the top surface4.
1 Formaldehyde-free versions are available.
2 But note that some of this work is used to re-flatten a wooden work surface after it has warped in some way.
3 Warpage of all kinds is reduced when pieces are cut to size, e.g. a noticeable bow on an 8' board may yield two 4' lengths each with only a slight bow. In this case your cupping will be far easier to deal with, and result in less loss of thickness, if you rip the boards into two (note: not necessarily right down the centre).
4 This has numerous advantages, but it's a lot more work.
