Seeing that you are not protecting what's under the paint then you don't necessarily to seal it. Acrylic paint is water resistant and should hold up fine for such a short period of time. The paint more likely would fade from the sun and water combination over time but seeing as its halloween props that can be adventageous. I use acrylic paint all the time and wouldn't want to buy the multitude of colors I use to weather props in house paint. That being said I some times give certain props and coat or two of a clear sealant which has always been more that adequate.

 

You could coat them with a clear Polycrylic spray.

 

I have a metal rain gauge with a hummingbird in flight on it. I painted the hummingbird with acrylic paints. It stays outside 24/7 until nighttime temperatures start getting close to freezing and it goes back out in the spring once temps stay above freezing. I live in the south so it’s outside more than in. The colors are still quite bright. They may have faded some but if so, it’s not obvious. I didn’t apply anything to protect it. It was painted maybe 3 years ago?

 

I'm less concerned with fading and more concerned with it just coming off. Thankfully, it's just a faux stone statue and a skeleton spider that I haven't decided if I really like yet. I've been wanting to re-do some props for a while so these were my test runs, but I forgot to account for the paint being craft-grade. I picked up a box of mixed craft paints for cheap at a local rummage sale so I wasn't as thorough in checking if they were outdoor safe nor do I know how old they are or how they were stored. I just happen to like how the statue is coming along and realized that if I did any more props that I should really think about how they'd hold up long-term.

 

If the acrylic paint isn't getting scratched, stressed, or absolutely baked in the sun it should actually be fine. I use that all the time and rarely have to touch-up.

 

I use Rust-Oleum for plastic to clear coat my plastic and resin props after repainting. And not a just light dusting. I do at least two coats on the entire prop to get a good seal. It protects against UV color fading too. Even when I use outdoor rated acrylics, they never last more than a few seasons on my plastic props before they start flaking.

I also coat the props I have not repainted to keep the factory paint and the plastic color from flaking or fading. I have had too many plastic pumpkins fade in the sun or the paint flake off, so it's just standard procedure for anything I buy now. Here is an HD prop I forgot to clear coat last year. Just six weeks in the elements did this.

I also helps greatly to spray a plastic primer on the props first if you are doing a full repaint, especially for things like tombstones and statues. A base of gray primer makes everything you paint over it stick better, then hit it with a few coats of flat clear to seal it all.

 

I use 2 or 3 coats of the Gloss version on my creatures

 

I'm in Mass and typically use a clear UV protection spray ... paint? I don't know if I can call it paint since it's clear. But you get the idea. It does help a lot with props that sit outside all of October and then live in a shed the rest of the year (with cycles of freeze, thaw, blazing summer, damp, etc).