Sanding filler smooth

I wish I came here before starting this job. Usually I'm pretty good with DIY but decorating is one thing I hate and try to avoid.

I removed the hand rail from the stairs and along with it some of the plaster. I used Uni Bond No More Cracks to fill it but thinking it would be reasonably easy to sand smooth didn't worry too much about getting a smooth finish. Bad mistake. I have tried sanding it down to blend in with the wall but cannot get the edges to blend in smoothly. Every time I try to sand the edges of the filler it crumbles away and leaves a slight edge which I know will look aweful once painted.

What is the best way of going about finishing this? I have tried sanding with a B&D Mouse and also a sanding block. The more I sand around the edges the more existing plaster gets removed which will leave the wall uneven. So what is the best way to sand it? Or would it be better to scrape it off and use something else? It's only a narrow band (about 10cm but runs the length of the wall)

I prefer to use a sharp scraper rather than sandpaper.

Some fillers (especially readymixed ones) set very hard and are difficult to smooth off. I like to use a fine finish plaster. Level it before it goes fully hard. No sense in overfilling, you'll only have to grind it away later. It helps to do a main fill leaving a surface slightly below level, then a thin one to finish up. That avoids sags and cracks in the final layer.

Hack the old filler out, then wet the area well, then refill. Allow to dry, should be much better, problem is the filler is not bonding with the wall.

Thanks for your suggestions guys. I've managed to sort it out. A friend recommended using a random orbital sander and this sanded it down (and smoothly) in minutes. I guess my mouse just wasn't up to the job. I keep your tips in mind for next time.

Perhaps Unibond should bring out a product called...'no more crumbly filler on the edges'