How ruff is it, do you have a piccy. Concrete screed will need to be a minimum of 40mm (preferably 60-70), and thats only if you use the correct additives like SBR or fibres to bond to to geither and seal the floor.
Be careful with self levelling anything, garage floors are designed to fall away from the house area, so if oil/fuel leak from a vehicle its away from the house.
Depends on hoe ruff it is, i would paint it with floor paint,it may take a few coats to get it nice and smooth
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