I've never repainted an enamel stove, so any suggestions are pure speculation on my part.......
I think that you could paint over the existing enamel, IF you roughed it up really well with say 80 grit sandpaper on a random orbital sander. I guess the question is how well the high-temperature stove paint will stick to the porceline enamel......
Thermolux gets consistantly great reviews for being a top-quality stove paint, maybe you could call them and ask them about painting over enamel?
Why are you painting it? Is the existing enamel in bad shape (i.e. cracked/chipped)? Or are you just wanting to change the color? The reason I ask is that if the enamel is badly chipped, painting it will only moderately cover that up as the surface will still be uneven after painting....
FWIW, if you choose to go the sandblasting route, I just picked up a sand blasting gun at HD for $17.99 You need a compressor to run it (at least a 6 gallon) and you basically put the blasting medium (sand, beads, crushed walnut shells, etc.) into a 5 gallon bucket and then a feeder hose drops into the bucket and sucks the sand into the gun while you're using it. I hooked it up the other day to test it before blasting an older Elm stove we just got--it actually worked pretty well! You definitely NEED to be outdoors, need goggles, a face mask, and gloves, and it works best if the gun has ceramic tips as the force of the sand will ruin metal tips pretty quickly.
Let us know how it works out!
NP