Sanding wall chases that have been extensively "feathered" on either side

[alligatorlizard]

Any tips for sanding these plastered-in wall chases? (pics below, of the same section before and after plastering)

The chases themselves are quite narrow, but the plasterer "feathered" them in with filler extending several inches on either side. I'm not quite sure how much needs sanding off. Do I just give it all a quick once over, and smooth out the transitions from feathered filler to original wall - or, do I need sand off all the feathering back to original wall and and then sand the filled-in chases till they're level?

Obviously the goal is to make the chases as un-noticeable as possible once painted. The plasterer said the feathering would help with this, and that it "just needed a bit of sanding". Seemed simple enough advice at the time, but now I've got around to actually doing it, seems like there's more than one approach, and I'm not sure which is best...

In terms of tools, I've started with a palm-sized sanding block - this has worked fine around the plug sockets (smoothing out the lumps and ridges so the sockets can be screwed in flush with the walls), but I think I'm going to need something bigger for the rest of the chase - either a larger hand sander, or a circular electric sander. One big consideration here is mess - I imagine the electric sander would be quicker, but I haven't used one before, and I'm guessing this would throw a lot more dust around than a manual sanding block? Or if I get one where you can connect a hoover (am using a Henry), could this actually be cleaner??

btw, I'm started with 120 grit, then a bit of 240. I did experiment with using 80 grit first, but this seemed to strip off too much of the feathered filler. Unless of course the feathered filler should all be stripped off...? Would rather not take that approach though, as that's going to be a LOT of dust!

Any pointers most welcome - as is probably obvious, this is my first time sanding plaster/filler... and have about 20 similar sized chases to do...

 

[aveatry]

80 is very aggressive. I would recommend one of these https://www.amazon.co.uk/Mirka- they are really good at sucking the dust up but remember to put a heppa bag in your henry and for best removal of dust stick the henry outside.
If you have other DIY jobs that may require a hoover in the future I would also recommend a wet and dry - I have a titan screwfix.

 

[alligatorlizard]

80 is very aggressive. I would recommend one of these https://www.amazon.co.uk/Mirka- they are really good at sucking the dust up but remember to put a heppa bag in your henry and for best removal of dust stick the henry outside.
If you have other DIY jobs that may require a hoover in the future I would also recommend a wet and dry - I have a titan screwfix.

Thanks, that Mirka looks like it might be the sort of thing I'm after. And presumably it would connect fine to a Henry? (and yes, am using the HEPA bags btw - not really practical to put it outside unfortunately - hopefully the bags are containing the worst of it...)

On a side note, I did look into Titan's, and Wickes wet&dry, but Henry's seemed to get more thumbs up overall so went with that - and I don't think the filters are much different in the others are they? None of them are class L/M/H?

 

[opps]

As per @aveatry 's post, the Mirka handy sanders are great, and I say that as a professional decorator with thousands of pounds worth of sanders. The abrasives are pricey though. Snide sheet are available.

 

[alligatorlizard]

Picked up a Mirka handy sander kit earlier today - it really does work! I'd expected it to still throw out a fair amount of dust at the sides, but as far as I can tell it gets the vast majority of all dust produced - maybe not 100%, but reduces it to a bit of fine dusting here and there as opposed to looking like someone emptied a bag of flour on the floor

I think I pretty much answered my other questions (re. how much to sand the feathering) once I started using this tool - just gently smoothing out what's there works best. The 120 pads are definitely the lowest grit needed here, quickly gets rid of any lumps/ridges, then I give it another quick pass with the 180.

The bits where the plasterer's must have got a bit lazy and not feathered it in as carefully at the sides (i.e. noticeable difference in surface level) are the hardest to smooth out - if I attack the raised edges too much, it starts to peel off and I'm just left with another noticeable transition but further in... If anyone's got any tips with smoothing the edges please let me know - but maybe I just need to get bit more of a feel for it (have sanded wood and car paint before, but plaster is somehow very different to work with!)

Anyway, thanks again for the recommendation - still a fair bit of work ahead, but at least now it all looks doable