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Welcome to the Bunnings Workshop community @Jacqueline23. It's sensational to have you join us, and many thanks for your question about levelling a slab.
I'd recommend using Lanko 20kg 173 Floor Leveller, and the primer for it is Lanko 1L Ultraprime 124 Surface Primer. Make sure the concrete is cleaned thoroughly with a pressure washer before priming. The floor leveller with do up to 25mm thick. Remember that outdoor slabs are typically graded and sloped away from the house so water runs off them. If you level them, you could cause water to pool if they are exposed to the weather.
Regarding the slabs being previously painted, is there paint remaining on them? If so, it must be removed for the primer and floor leveller to adhere. I am unaware of a primer that will adhere to a painted surface for overcoating with a cement product.
For the cracks, you can use Gripset Betta 1L Concrete Crack Filler. However, if the cracks are fine, then the floor leveller will fill them for you.
Please let me know if you have any questions.
Mitchell
There's no particularly easy way to remove paint from concrete @Jacqueline23. In some instances, it might be easiest to have the slab ground back and re-surfaced by removing material.
How badly do the slabs need to be levelled?
Mitchell
I've done some investigating @Jacqueline23, and if it is water-based paint on the slab, then you can prime it with Sikafloor -14 prep and prime and level with Lanko 20kg 133 Pro Level Floor Leveller. However, if it is a solvent-based paint, it must be abraided and removed. To test if it is water-based, apply some methylated spirits to a rag and wipe it over the paint. If the cloth becomes stained with the paint colour, it is a water-based paint.
The Sikafloor -14 prep and prime is a somewhat specialised product and must be ordered for you through your nearest store's Special Orders team.
Mitchell
