A little trick Ive used:

Thread a length of fishing line through a small button with a
needle. Carefully punch the needle through the inside of a condom
so the button is inside. A drop of latex will seal the tiny hole
the needle made.

Fill the condom with blood and seal it.

Attach the condom to a heavy (but small) piece of cardboard with
super glue or liquid latex. With a bit of twine, tie this
cardboard backing to your actor.

Now score the costume where you want the blood to fly. Thread the
fishing line through the costume and set up your shot. When you
yank hard on the fishing line, the button will pull out of the
condom and the blood will spray.

To make it "explode" use an air compressor. I use a tube made from
a length (about 18 inches long) of PVC pipe and attach it to the
end of the compressor hose. Put a small, red sponge in the bottom
- it acts as a pressure point and keeps the mixture in place. With
a little work and experimentation you can place the tube behind
the actor and "pow"! when you hit the valve the air forces
everything out.

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The aim of an argument or discussion should not be victory, but progress.
Joseph Joubert, essayist (1754-1824)

Another thought, if the zombie has been dead awhile you can use flour or dirt puffs instead of blood. Far neater and the effect can look really nice on film. After all, if the zombies been walking around awhile the blood will pool in the feet rather than circulate.

RJSchwarz

As always, there is another way.

I'm currently in post with a zombie film myself and some friends made.

We managed to turn two disadvantages (a v.low res camera & lack of money) into a positive - we skipped using squibs altogether, although we did slather home made blood (corn syrup, dye, etc) all over faces/fronts of clothing.

Now, in post(AE), I'm adding in blood sprays, blood pools, etc. I'm messing with contrast, levels, curves etc to essentially hid the difference between our raw footage and the nice clean colors of the FX.

Its a fair amount of work to add onto your Post schedule, but it meant that film was very smooth, quick and clean - no fiddly squibs, no waiting for them to be set up, and we coud shoot as man runs as we liked (we kept it down to 3-5 takes max).

Essentially, we used digital post production to wipe out an entire layer of complexity and cost (time/money/patience of volunteers) during actual production.

We're nearing Finals, hopefully by this Friday. Ill post a link once done and you can judged for yourselves.

Duel

Fix it in post.