Hello Roy and welcome...
Not all levellers are suitable for external use...
These are a few options that are available use...
Fast-setting cement based levelling compound for internal and external use from 3 to 20 mm thick.
Applications:
Internal and external levelling compound for all normal supports used in building provided they are clean, such as concrete slabs, cement screeds, old ceramic flooring, terrazzo tile, natural stone.
Apply 3 to 10 mm thick per coat.Coverage
1,5 kg/m2 per mm of thickness.Packaging
25 kg bags.
ARDEX K 301 - Fast Setting Exterior Self-Smoothing Levelling and Resurfacing Compound
Size (pack): 25kg
Coverage: Approximately 1.6kg ARDEX K 301 powder/m²/ mm, i.e. approximately 7.8m² at 2mm thickness per 25kg bag.
ARDEX K 301 Exterior Self-Smoothing Levelling and Resurfacing Compound is a special fast setting cement-based product designed to smooth and level rough and damaged concrete surfaces in external and internal situations. ARDEX K 301 is a grey powder consisting of special cements, graded aggregates and high quality synthetic resins. When mixed with water, a fluid mortar is produced which sets within 2 hours and can normally be walked on after approximately 2-3 hours at 20°C. ARDEX K 301 can be applied from 2mm to 20mm in thickness. The set and hardened ARDEX K 301 is usually ready to receive light wheeled traffic after 48 hours at 20°C.
What tile do you intend to use..?