tile on tiles

Got a bit of tiling to do some of which is on top of some old tiles. They are sound, anything I should do to them first or just carry on straight over them. They were already tiled over but I have managed to remove the top layer cleanly ready to start again! Also got to put some on a patch that has just got render on it and not skimmed, should I PVA this first or will the adhesive be fine without it. Many thanks
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It won't hurt (dilute.) Also splash some on the tiles with a paint brush. Just flick it from a jam jar or whatever. Only small droplets are what you need otherwise the drops will run off.
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