This is how I'd recommend you handle the mold on your patio.
Remove all existing paint that is visibly present.Begin by renting a commercial piece of equipment from your local home improvement store.
- The current paint has to be removed in its entirety by either chipping, grinding or fully stripping away the paint to restore the concrete to its original color.
- The thickness of the existing paint will determine the method used to remove the paint. This process may take a day.
Treat the mold. It needs to be eliminated. A bleach based solution and a pressure washer will eliminate the mold. One can choose to create a solution simply by using a one-to-one ratio bleach and water solution. Pressure washers can be rented at one's local home improvement center at a reasonable cost.
Seal. Once the area has been cleared of all dust and debris the concrete is ready for sealing. This final step is a two part process and one must check with specific product guidelines to be sure enough product is available to coat the entire area twice.
- I recommend Rustoleum or Westcoat specialty coating systems with a UV rating. This is especially important if the patio is exposed to a substantial amount of sunlight.
- Should one decide to pursue an alternative product, be sure it is an epoxy based substance. I recommend epoxy based products over a concrete sealant because over time sealants tend to crack and peel.
- To begin sealing the area apply the epoxy product in a North to South application. Be sure to check the manufacturer instructions for drying times and conditions.
Reapply. After the allowed recommended drying time reapply the product in an East to West application. The concrete patio should now be ready to enjoy!
I hope I have fully answered your question. Please feel free to post any additional questions in regards to this project.