To make short crust pastry in a food processor, which blade should I use? The sharp steel one or the blunt plastic one?

Agree w Cynthia! We use only paddle in a mixer for our doughs and haven't used a food processor before. Paddle gives you more control of butter we find.

The best result may come from using the paddle attachment on a KithenAid mixer. Granted, short dough is very forgiving because of its high ratio of buttter; nonetheless, the more it is mixed at higher rates of speed, the softer that butter becomes, resulting in a dough that is not unlike bubble gum. It is going to need to be chilled before rolling/shaping, but the time may be reduced and the texture less compromised by using a mixer rather than a food processor.

I have used both. The steel blade gives me better results.

Nothing to do with shortcrust, which I have not tried in the food processor, but I have found that the plastic blade can be great for other types of dough that require lots of mixing - I often use mine to make gougeres. While it might not work for pie dough, don't discount it yet!

I agree I tucked mine away and never use it, always use the steel blade it only takes a few short pulses to get the pie dough to where you want it.

When the folks at Cook's Illustrated tested food processors, they found the plastic dough blade not very functional. Use the metal.