About 20 years ago I used a water based (unusual for then) clear sealer on blockwork inside a lorry steam cleaning area. It was bomb proof. I don't think the company exists now but was based in the Ayrshire area. The product must still be around somewhere. If I remember, but don't hold your breath, I will get back. An alternative may be anti-graffiti paint which is clear but may be more like toffee. Best wishes.
