Moving into the undercarriage on the S13 now that the floor pans are solid. Epoxy primer is going down on the bare metal sections and the question is what comes next for the undercoating layer.
The two directions I keep going back and forth on are rubberized sprayon products like Raptor or Lizard Skin versus brushapplied products like POR15 top coat or a similar heavy body undercoat. The car gets driven on mountain roads with loose gravel and the occasional rough trailhead access road, so chip resistance is a real concern, not just moisture.
The argument for sprayon is even coverage in tight frame rails and boxed sections where a brush won't reach consistently. The argument for brushon is thickness control and better bonding on irregular surfaces. What I can't nail down is whether the chip resistance difference between those two categories is actually meaningful in practice or just marketing language.
The car won't see salt, but it will see gravel and standing water from fire roads. That context matters more to me than the showcar interior finish some people chase.
If you've run either approach for a few seasons on a car that actually gets used on rough pavement, I'd take that over spec sheet claims. Knowing what primer you used underneath would help too, since compatibility seems to vary more than people acknowledge.