What materials work best for painting the Marvel United figures? I have experience painting plastic (Warhammer) and resin models.
Like most CMON minis the Marvel United figures are made of a semi-flexible PVC.
While washing the figures is not strictly needed they may have left over mold release agent on them, which can make it difficult for paint to adhere to the surface. Therefore to ensure the best results you can use warm water and a mild soap to clean them, making sure to allow them to dry completely before painting.
Acrylic paints will adhere to CMON's PVC well enough that if un-thinned paint is used no primer is required. If paint is thinned for layering or airbrushing it's essential to allow each layer to dry before moving onto the next one.
If thinned paints, washes or inks are to be used priming is required, but some primers work better than others, Vallejo Auxiliary or Hobby Paint, Army Painter Color Primer, Krylon Dual Paint + Primer, Duplicolor Sandable, or Rust-oleam Painter’s Touch Ultra Cover 2x all work well. Citadel's Base paints should work, but their sprays can stay tacky on PVC.
After painting a clear coat should be applied to help protect the painted surface and prevent the paint from chipping or fading over time.