I would spread with a rubber squeegee and follow up with a roller.
If the slab is in direct sunlight I would apply it as the temperature is going down. I have seen the expanding air coming from the concrete create small bubbles from the heat after being applied. I don't think it would be too detrimental but might have not sealed as well because of it. This was years ago and I know nothing of the product you are using, so just a thought.
