I'M thinking of making some rocks following this recipe: 1 part Portland cement, 1 part peat, 1 part perlite or vermiculite, 1 part sand. After a long curing time, the instructions say to let the piece sit out in the rain for several weeks to leech out the lime. Does it sound like something I would need to seal after curing? If so, any advice as to what to seal it with? I'm going for a matte finish.