I did a fair bit of this over the years. Sand and cement? Mix, mix, and mix again. It is astonishing how much water can be retained in the sand (by hygroscopic action?) and then, as you try to finish, it all comes to the surface and makes your finished wall surface look like runny Brie...
I used to 'throw' the so-called "dry mix" from one mixing board to another, then back. adding water was "little and infrequent". For the relatively tiny areas (under 4sqft) that I was doing, this method was quick and effective.
Offending hollow render hacked back to brick (irregular edge, oblong-ish hole,
rounded corners, helps) then P.V.A. watered down as prescribed. If you mix the sand/cement as the P.V.A. is going off, it should be nearly right for 'scudding' on an initial coat of render as you finish mixing. Twenty two ways of doing that, scudding I mean, but I used to prefer a VERY dry mix in handfuls (use builder's gloves) thrown hard into the cavity, with a throwing board underneath to catch 'ricochets'. The P.V.A. moisture amalgamated with the cement hydration, so caution is needed at this stage. I usually tried to run a straight edge over the hole, immediately afterwards, just to knock of any obvious hillocks of impacted render. Step back, clean up, leave. Cuppa, check phone messages, walk dog, what-have-you, until render has changed colour. That means water is evaporating, cement is setting just enough for Stage Two. It is even better if you can leave it 'till tomorrow.
Stage Two: dry-ish mix, as before, plasterer's hawk and float help, but I've got by with a piece of ply and a rough chunk of 2"x4" loose in my hand.
Starting from the bottom of the hole, a smooth application of dry-ish mix into the remaining cavity. N.B. the depth here may be varying from under a gnat's todger to finger-thickness and the scudding coat nay be fragile, so go gently. Sand and cement and gentle in the same sentence? You know what I mean. No gym-bunny antics. Pressing hard with the float or "tubafor" will bring the retained moisture to the surface and your surface will resemble Mick Jagger's complexion before he smears on the hemorrhoid cream to tighten it all up.
Once in, use a 2"x4" bridging across the patch and walk it up the wall, oscillating from side-to-side, GENTLY. DON'T RINSE, or repeat. You should now have a patch relatively smooth and in the plane of the wall, but possibly bleeding moisture. Surfacing it should be done with a DRY sponge, vertically tapped onto the surface, taking OFF the appeared moisture....or a knackered old metal brush applied gently two hours later, or...whatever floats yer boat - see above.
I found a dry sponge - think kitchen pan scrubber, rather than builder's sponge - carefully applied (patted, not smeared) at the edges, allowing 'blending in along the edges'. It took time to get the 'right touch', but it was worth a bit of experimentation on a patch away from the main frontage, to be seen by all.
All done? Nope, you CAN wait a week, then use 60 or 80 grade sandpaper to blend in further. Your choice. I found even a kitchen scrubber (the one you wrecked with cement earlier) turned round to the green side was enough to knock off some of the 'green' cement after 2-3 days and you could GENTLY blend in before leaving it to go off for about 10 days before painting.
H.T.H. if you have never mixed cement before. It's not hard, but there are wee wrinkles and pit-falls. I've done my best to touch on all of them. I didn't set out to write a "Dummies Guide", but it just happened.
S.
Happy Easter, Everyone!
