With used equipment you never know what might have happened through its life. I have used the following processes on lighter fences such as yours.
After first making sure the fence head lock is clean and consistently functional I would next work on the rail and fence.
1 - Take a good straightedge and check the rail the fence rides and locks on. If the rail isn't straight your fence isn't going to be straight. If it isn't straight you may be able to straighten it out by muscling it out on your bench vice. Or place blocks at the ends and with a beefy board and a clamp in the middle squeeze it in to bow it back straight. Or place it on your bench top and again with the blocks supporting the ends and a clamp in the middle try to get the bow out. When you mount it back on your table saw
2 - Make sure the fence is straight using the straightedge. If there is a bow in it use the same method as for the rail to straighten it out.
3 - Check to make sure the fence face is perpendicular to the tablesaw top and isn't twisted. Lock the fence on your tablesaw. Take your square and check the fence face against the tablesaw top front, middle and back. If they are all consistently out of alignment adjust the fence as your fence allows, you may have to use shims at the bolt holes. I like to use an aluminum pop or beer can because you can easily cut shims out with scissors and a circle punch. If there is a twist in the fence hold one end in a vise and use a clamp at the free end to tweak the twist out, then check for straightness again.
4 - Now you can adjust your fence's parallelism to the blade.
Like some of the others have replied I prefer MDF sealed with shellac.
Whittier, CA, USA
Dona nobis pacem