Jimmy, if you really want it dead smooth then you'll need to use a self-levelling compound. Google 'F Ball' and have a look at their products. I'd imagine you'd need something like their Stopgap 300HD product for your application, but if you give their technical department a call I'm sure they'll help you out. You need to work out what the variation in your floor is, you can usually get around 3-10mm using a self-levelling compound applied with a trowel, if your floor is more uneven than that you might need to get it pump applied. Also, you will need some decent floor paint as these sort of compounds are not designed to be a wearing surface. I used to work for a flooring contractor and can still just about remember the technical stuff...
