floor screed is pretty good stuff you mix it with laytex and float it like plaster. however under the pressure of a car jack or axle stands it will crack and shatter still. concrete is the answer if you want to do anything heavy.. you could screed then carpet or lino it which may help.
on the paint side of things . use the 2 pack floor paint, this is desighned for factory floors that have stacker trucks driving on it so it wont crack under pressure. mix up. cut in roll out........swiftly
to be honest i would say roll out some cheap carpet or lino. cheap and easy and warmer to even if its a bit lumpy. or just work on ya car/bike in the front room like i do. and put ya parts in the dishwasher to clean em.
i do have a very understanding girlfriend though
