Cosplay tip!
I'm really tired right now, but I've been looking at a bunch of cosplay tutorials and I've yet to see anyone mention Aleene's Fabric Fusion as a sealant/top coat. It's a glue meant for fabric, but I've used it for everything from craft foam to pax paint covered latex horns. Basically all you have to do is get a paint brush and cover whatever you need to be shiny with a layer (watered down if you don't want brush strokes) and let it dry for a day or so. BAM! Super shiny but extremely flexible protective coating. The texture lends to a weird slickness, but it's super easy to clean dirt off of and so far seems pretty unreactive to anything. I've used it on craft foam, acrylic, latex, and plastidip and there's been no noticable chemical reactions. The only cons I've found so far is the wait time for the glue to dry (a hastle if you're doing watered down layers) and the cost. It's about twice as much as mod podge where I am. However, for small pieces that absolutely need to be shiny and flexible, it's invaluable!