can be oil- or water-based, and you can use either kind.
Polyurethane is a clear varnish that dries to a hard, solid finish. Polyurethane can be oil- or water-based, and you can use either kind over latex paint; however, oil-based polyurethane tends to yellow as it ages, whereas water-based varieties stay transparent. Assuming you want.
I bought a CH HVLP HV, I am spraying latex paint on wardrobe doors. I’m wondering about achieving a much smoother finish by applying a polyurethane top coat. What latex paint do you mean, specifically?