(I've typed this so much I should just make another tutorial on FigureRealm)
First lets confirm what sealer does.
The main benefit of sealing action figures is protect the paintjob to make it safe to handle them. The slick surface also makes it easier to wipe them clean.
That's it. It's not bulletproof. If you throw the figure into a box with other figures, the paint is gonna get damaged anyway.
Some paint brands don't need sealer. Some paints, within the same line of paints, have different levels of durability, so after you're done painting use your best judgement if it needs sealer.
In my experience, glossy paints usually don't need sealer (Testors, Tamiya). Flat colors tend to need it, simply because dust likes to stick to flat colors and it's so
ing annoying trying to clean them.
First I use Liquitex Acrylic Varnish to seal things. If it's too glossy, I mix in a little Vallejo Matte Acrylic Varnish to kill the shine. I use the Matte straight too, for things like old rubber, terrain, etc.
These are brush-on sealers, spray sealers might react badly with the acrylic paint. I've had good results with Testors DullCote, but haven't used it in years, so who knows if it's still the same formula.
Topics on the sealers I use:
www.figurerealm.com/topic?topicid=9368
I've wondered about using the kind of sealer people use on hardwood floors, I imagine that should be mega durable. However, I'm not sure how thick it is or how it would react to acrylic paint. There are all sorts of sealers, polyurethane, lacquer, enamel, latex, synthetic, etc.
