I find one of the most important part of engraving slate coasters is the prep.

When I take the coaster out of its packaging, I use a coarse scuffing pad and scuff up the top, (not too much, just enough to remove the top part. 

I use my air compressor to blow away any loose powder (canned air will work good too) then I wipe the coaster down with a DRY lint free micro fibre cloth and give it a quick shot of air again.

I've tried several different clear coat sprays but didn't really care for the finish on any of them, as a lot of them I found looked flaky after they were engraved. My neighbor, who is a body man, gave me a can of flat black automotive spray paint and it gave me the most amazing definition on my coasters. You should be able to get this at an auto parts supply or from a body shop. I recommend, although I can't say what brand will be available in your area but just make sure it is flat for the best definition and makes your image pop. 

I only use one light coat and let it dry for at least half an hour.

Be sure to invert your image (the one in the file is inverted already for you)

Frame and process your slate.