If you are repainting you shouldn't need a primer, unless you've stripped it down to the mdf. If old finish is still sound, just sand lightly and repaint.
Don't use a water based primer like Gardz on bare mdf. Zinser oil based would be my choice. The shellac idea is to beat the "fuzzys" that bare mdf gets when sanded or machined. The fibers are soft and don't sand off. Shellac or laquer dries fast and stiffens the fuzzys so they sand off.
[CENTER][FONT=Comic Sans MS]Measure twice, cut once.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]Look at the nail, not the hammer. Watch the fence, not the blade.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.[/FONT][/CENTER]