What can I use instead of undercoat?

What you can use instead of undercoat depends on whether you mean painting walls or painting car. For house painting, instead of undercoat which is also called primer or undercoat paint, you can use primer, primer sealer, or self-priming paint which is paint and primer in one, which can skip separate undercoat if walls are in good condition and similar color. For example, if painting over clean previously painted walls in good condition, you can use two coats of quality self-priming paint instead of separate undercoat. If surface is bare drywall, wood, or stained, you should still use primer for best adhesion, but you can use multi-purpose primer or bonding primer instead of specific undercoat. For car painting, instead of undercoat, you can use primer, primer surfacer, epoxy primer, or self-etching primer, depending on bare metal or filler. If you mean automotive undercoat for rust protection underneath car, instead of rubberized undercoat, you can use waxoyl, rubberized coating, polyurethane coating, or rust proofing spray. So instead of undercoat, you can generally use primer, primer sealer, self-priming paint, or direct to surface paint with good prep, but for bare or problematic surfaces primer is still recommended for durability.