I airbrush so I have to use a sealer or the paint would scratch too easy.
I like to use Testor's Dullcote because it has NEVER gone tacky on me. A lot of model horse artist's will use something like Krylon Workable Fixatif (yes that's a weird spelling!) because it's a nice protective clear coat and then give a light coat of Dullcote as a final sealer to prevent tacky issues. The Dullcote IS very dull though, so don't be expecting a shiny finish unless you just very, very lightly spray it.
Also, to prevent tackiness I always wait at least a day after I finish painting to seal a model with Fixatif. And then I wait another day to use Dullcote. Then the next day I gloss the eye/hooves with a clear acrylic paint (the little bottles in the craft section of acrylic paint is pretty much all I use, and there is a clear version as well which is perfect for eyeballs!)
For a primer, I usually start out with a very light coat Rustoleum Plastic Primer, let it dry a few days, and then Krylon Gesso Spray. You could probably get away with just the Gesso. Picking the right primer is just as important as picking the right sealer. Primers can cause tacky issues too.
Gesso has always proven safe for me. The Krylon Gesso Spray can be a little hard to find, I usually mail order it, but a good art supply store should have it. If you are hand painting and don't mind the brush strokes, even Walmart carries gesso that you can paint on by hand. (They also carry the Krylon Workable Fixatif).