How do I clean a Hammerite paint brush?
The consensus of opinion on the Ultimate Handyman forum seems to be to use an old brush and throw it away after use.

A couple of alternatives are suggested:


  Ordianry cellulose thinners will also work.


So presumably the Hammerite product is cellulose thinners based.

and


  I wrap mine very tightly in plastic and stick them in my brushmate, as long as i use them every couple of months they stay reasonably soft, even if they go a little stiff a drop of hammerite thiners brings them back to life.


Which is something I do overnight (or a couple of days at most) when using gloss paint. It does keep the bristles soft for a while so saves on white spirit and having to find a clean brush for the next coat.