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I was asked to paint a fair-sized concrete statue a year ago and have recently finished it. However, I didn't research the paint required in-depth, and as such have used standard, what I assume is chemical based paint for interior use as opposed to for exterior use. We live in a country with a lot of rain and it's intended for exterior use.
It's been primed in 'B&Q White Masonry Primer and undercoat for exterior concrete, brick and stone'. It was cleaned beforehand using water and a small amount of fairy liquid, with toothbrush and sponge, and then rinsed with the hose. Any imperfections, cracks, and holes I filled with Terracotta Milliput and sanded to remove most of an existing mold line and smooth out the added material. The statue was already 2 years old when I got it. I've been looking over the internet but admittedly confusedly, but maybe I'm in denial and will need to start over.

Would the paint immediately peel, would I have to start over, and what - if any - sealant would I require or is recommended?

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Interior-grade paint can sometimes be affected if it gets wet (depending on the composition), and may fade or degrade from exposure to UV (I'm not familiar with how the specific paint you used will hold up to exterior exposure and weather). Normally, I wouldn't worry about peeling, given the prep you describe. But concrete can absorb moisture, which can degrade the paint's bond. If a lot of work went into detailed painting, I'd be tempted to just protect it with an exterior-grade clear coat, like one designed for car finishes. You can get the clear coat in spray cans, which will be good for this purpose.

Make sure the statue is good and dry throughout (concrete is porous and holds moisture). Ideally, this should have been done before painting with acrylic, but drying out the concrete with a slow process should allow water vapor to escape. For four days to a week, keep it in a closed room with:

  • A space heater that maintains the room as hot as the hottest outdoor temperature the statue will see (but at least in the mid-80s F).
  • A dehumidifier set at a low humidity setting (like 35% or 40% RH).

Then allow the room to cool to a temperature you can comfortably work in (and within the recommended application temperature for the clear coat). But keep the dehumidifier running (especially important when the room cools because the humidity will spike at the lower temperature). Seal the bottom of the statue so it doesn't absorb moisture from there. When that is completely dry/cured, depending on what you use, stand the statue up to clear coat it.

Apply the clear coat in many light layers (about four should do it). Spray from different angles and inspect for coverage with a light after each coat, especially in nooks and crannies that may not get good coverage. Follow the product instructions for how long to wait between coats.

If you start with very dry concrete and encase the statue in a good, continuous plastic shell, the paint should hold up to the weather. When you transport the statue, protect it so that the clear coat doesn't get damaged.

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