For a hard, scratch-resistant, waterproof, and dirt-repellent top coating for interior and exterior surfaces like plaster walls and cement board siding, I would recommend considering a clear protective coating such as a polyurethane or an epoxy sealer. Both options provide excellent durability and protection.
Polyurethane: Polyurethane is a clear, protective coating that is commonly used on wood surfaces, but it can also be applied to other materials like plaster walls and cement board siding. It creates a hard, scratch-resistant surface and offers good water resistance. Look for a polyurethane specifically designed for exterior use if you plan to apply it to the siding. It's important to note that polyurethane may slightly darken or yellow over time, so it's advisable to test it on a small inconspicuous area before applying it to the entire surface.
Epoxy sealer: Epoxy sealers provide a strong, durable, and waterproof finish. They are commonly used on garage floors and countertops but can also be applied to other surfaces. Epoxy sealers are available in clear formulations and can provide excellent protection against scratches, dings, and moisture. However, they may require a bit more preparation and application expertise compared to polyurethane.
Before applying any clear protective coating, make sure to clean the surface thoroughly and ensure that the existing latex paint is in good condition. Follow the manufacturer's instructions for proper application, including any surface preparation, recommended tools, and curing time.
It's always a good idea to test the chosen coating on a small inconspicuous area before applying it to the entire surface to ensure compatibility with the existing latex paint and to verify the desired level of scratch and ding resistance.
Lastly, keep in mind that while these coatings can provide an added layer of protection, no coating is completely indestructible. Proper maintenance and care, such as avoiding abrasive cleaners and harsh impacts, will help prolong the longevity of the coating and keep the surfaces looking their best.