you never mentioned if your porch is smooth or rough. Though I've not tried this method myself I've heard putting a wash of straight vinegar on smooth concrete will etch it and help the paint adhere. Again, I've not tried this myself but it does make sense to me. Also, if you decide to use a primer of some kind, get it tinted as close to the colour of your concrete as possible. I don't think the problem will go away 100%, but at least if it wears down to the primer, you won't have white showing through.