I got some of this in the post today it's like rocking horse poo over here to get so I've never used it before so am I right in saying that 500 is the finest of the 2 types and if you brush it on does it need any sanding any tips much appreicated

Actually the 1000 is the finest of the two. I use 500 to fill small airholes in resin, ejector pin marks, and small gaps. Sometimes you can fill areas that are a little bit bigger by brushing on a layer , letting it dry (just a minute or 2) then brushing on another layer till you get the result you are looking for. You can sand it if you need to, it depends on how you have applied it. The 1000 I use to fill in around grab handles on AFV's, also to check for gaps in some places where it's hard to tell if there is one. I have also used the 1000 to create a cast texture by brushing it on, then using an old stiff brush to stiple it before it dries. Be sure to use it in a well ventilated area, the odor is potent, and you can use laquer thinner to clean your brush.

HTH,
Mike