Have you ever had a Safari, Papo, Collecta or similar brand model that ended up sticky and tacky after you painted and sealed it?
This is because of a chemical reaction between the type of material used to make the model and the paints or sealers.
One thing that I learned from a collector and painter of My Little Pony Brand models, is to use Mod Podge after the final sealing. Mod Podge stops the chemical break down (not sure of the science) and leaves a glossy or matte finish on the model depending on the Mod Podge you use.
You paint the Mod Podge on with a paint brush and let it dry (it dries quickly). Don't glop it on or it will leave blobs of white - just lightly brush it on and let it dry. You can do multiple layers. It will not take away from the paint job or anything.
The two bottles of Mod Podge I have, have lasted me for several years. I have used them on many models - see some of the pictures below.