hi roger
how long has the concrete been down???
if its a while, then it will be very dry and very porous too!!
give it a quick scrape if there are any high spots and a good brush...sprinkle some water down too to grab the dust and put a bit of moisture into it (only if its not recently laid
)
i would use sbr to prime and then hit it with your slc
mix it in a gorilla tub and pour ...dont hang about too much, pour spread out with a trowel or squeegee etc and when you have done a section use a spiked roller to take any air bubbles out.....then onto the next section till done
you can pick a cheap spiked roller up from ebay for around £19-00
make the mix so it runs easily, let it do the work if you get my point
ed