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I used gyproc easifill on a particularly beefy crack, probably up to 50mm deep in places. But only because it's what I had to hand. I am certainly one whose advice should be taken with all appropriate "I'm just having a go" caveats. But I built it up in a few layers and it doesn't seem to have crumbled, cracked or come apart in 6 or 7 months.
I like easifill as it seems easy to apply, has a decent drying time (given dryish conditions) and sands well. But it does sink back quite a bit. Especially the ready mixed stuff as it's fairly wet so there's a good bit of moisture/volume lost in the drying out (is my assumption). But, so long as I expect to refill/cover at least once, it does what I want it to from a DIY-er perspective.
Thanks!
I did try various searches in the thread, and I thought I remembered @stevo_com in particular doing this very task, but I couldn't find anything specific.
I've just watched a Charlie DIYte video about the same job and he also recommends the PVA priming.