Repairing fresh skim of moldy rough cement window sill.

[medusa1569]

I am a renter and I enjoy feeding the birds by depositing tidbits of rice on the cement sill of the 60yr+ apartment building. I've repainted the sill before with no problem but now I'd like to deal with the problem of the reappearing mold mostly due to the deep sponge like holds that the sill has. NO holes..less of a problem to maintain. I'd like to do a skim coat on the sill and I'm assuming any cement patch will do but I am also wondering if there is an additive for prevention of mold growth. I'd like to make the skim coat as smooth and slick as possible to avoid further moisture issues. What is the way to achieve the smoothest less gritty finish?