Alf, it's a liquid. Westley's Bleche White
I'm a member on a model building forum. There's been questions regarding Bleche White alternatives, mainly because some Canadians can't find it. Now, the topic has to do with removing models from molds... a cleanup cure for mold release. This is what they said:
Westley's Bleche White is little more than a liquid trisodium phosphate solution (TSP). TSP is a phosphate detergent, used a lot in furniture refinishing, as well as household cleaners. Check your local supermarket for "Spic and Span", an old-line powdered cleaning detergent--this is almost pure TSP, and a strong solution of it should do the trick for you as well.
Another option is Naptha, which is a paint thinner (also cigarette lighter fluid!), that just about any paint store, or paint department of a home improvement store (or even Walmart!) carries. It degreases resin like "RIGHT NOW", and short dunks of resin into it won't hurt the resin either. This does need to be used in an area AWAY from open flame, preferably outdoors.
Personally I've had resin that Westley's didn't remove all the mold release. Easy Off heavy-duty oven cleaner works wonders. It's nasty stuff, but it will make your models squeaky clean and able to be painted properly.
If you use any of the aforementioned chemicals, I'd definitely want to test in an inconspicuous spot, some of these MAY dull the gel coat.