Sand it to key the varnish, ideally you'd use some sort of adhesion primer too, I like Zinsser BIN which is shellac based so will stick to anything.

Now, on the advice on several websites and my friend you cannot leave bare chalk paint.

To me this begs the question, why use it then?

Now, on the advice on several websites and my friend you cannot leave bare chalk paint. You need to have some sort of finish on top of it. Several of the tutorials talk about using wax / paste wax.

With deference to any of them, I think they've missed the point. Unless its particular qualities are what you want to see in the finished item why use chalk paint in the first place?

This same argument extends almost universally to all finishes — why do you choose to use any product? For example, if you want the protection provided by varnish you use varnish, if you want the low-sheen 'natural wood' look of an oiled finish you use oil.

With paint it's not much different. You pick — or that should be, you should pick — based mainly on the look and level of protection you require (with some financial and practical considerations thrown in as well of course).

Varnishing chalk paint is basically just silly, unless there are other reasons to be using that paint to begin with, e.g. to use up some remaining chalk paint before it dries in the tin. But as a general principle, they make paints that look, feel and perform much the same as varnished chalk paint, without the need to overcoat with another product. It just makes sense, both money- and time-wise to use a product like that instead.

In short, I am using chalk paint on a table that would very well see some wear and tear. More than likely it will come into contact with liquids. I was considering using wax as a top coat but now I am not so sure. In this scenario what should I consider using for a top coat finish?

Wax is, as that quote you included rightly says, a very poor protective finish.

Wax always has been and always will be primarily a surface polish. Unfortunately that fact has become a little buried in recent years, partly because of the finishing industry's use of flowery (read: inaccurate) language when promoting their products. Wax adds shine and a modicum of extra water-resistance because it makes watery liquids bead up on the surface, just like on a well-waxed car, but this is not actual waterproofing because in actual fact wax is quite porous. If you want a truly water-resistant surface on wood you use varnish or lacquer.

To summarise my recommendations:

• use chalk paint for the look of chalk paint;
• if you need a more durable surface simply don't use chalk paint.