Why is my pastry blistering in the microwave? I am trying to make my own shortcrust pastry, but don't have a blender or food mixer. I also don't have a gas or electric oven and am using the convection bake function of a microwave.

I mix 1 1/4 cups of flour, 1/2 cup of butter (grated with a cheese grater then back in the fridge before finally mixing into the flour), 1/4 tsp sugar &amp; 1/4 tsp salt, and about 5 tbs of ice water. Then I convection-bake it in the microwave.

Somehow I just can't get it right. I tried blind baking and my pastry ended up looking like a very pimply kid with blisters all over. It was also sweating butter or something a fair bit when it was baking. It tastes okay, but a bit crunchy like a cookie, and impossible to cut without breaking apart.

What am I doing wrong? Is it the temperature (190 degree Celsius/375 F)? Is it because I grated my butter? Too much water? The way I roll my pastry? Any advice is really appreciated as I'm a total newbie to baking and don't understand a whole lot about it yet.
This is because you are using a microwave. In theory, if you could turn off the microwaves in the microwave oven, you could use the convection function to bake things. In practice, we have had several questions which indicate that this is not how convection microwaves work. They keep nuking your dough, making it inedible. In your case, they are cooking all the moisture locally, resulting in mini-steam-explosions which create blisters, and a dried out, hard and crispy dough. 

I am sorry, but there is no way you can bake with a microwave, not short pastry, and not other things. One exception might be quickly eaten "microwave cupcakes" which don't have much flour to start with and are baked for a very short time. I have seen people claim that they work, but never actually tried them. 

For baking, you need a non-microwaving oven. The cheapest solution is a toaster oven with a 30x30 cm inner size, they are somewhat trickier to use than a big oven, but good enough for a small budget.