Before you can put the casting in a clients home or out in their garden you should use a concrete waterproofing sealer. Unsealed concrete will develop an ugly chalk like surface as salts develop. A good concrete sealer should control this dusting. I have used three different products. A concrete densifier/sealer an acrylic treatment like Drylock and a $100 per gallon masonry waterproofer/sealer.

The V-Seal 117 densifier claims to react with the salts in the concrete and create a harder and weatherproof surface(It claims to make concrete waterproof as the salts are modified and fill the pores in the casting). Highways are finished with this product, the floors at Home Depot have it too. If you are going to polish the finished casting then this is a mandatory step as it permanently stops dusting.

Sealer Choice Gold is a product that will seal stone products for a 15 year finish. The product is rated for interior and exterior use and makes an excellent concrete waterproof sealer. The only issue that is unclear is how it handles dusting in concrete. It imparts almost no sheen to the concrete. Very desirable.

My go to choice is the Dryloc Masonary Treatment. It is cheap and gives the casting a finished look. Use a bit of steel wool and wax and it even feels great.