Primers, Sealers, and Undercoats
What this domain tests
Primer types, surface preparation, film build, flash-off, sanding, sealing, and isolating substrates.
Core rules
- Match primer to substrate and purpose: etch primer for bare steel and aluminium, epoxy for corrosion protection and bare metal, urethane primer-surfacer for filling and sanding, and sealer for colour hold-out and isolating old finishes.
- Apply within the specified dry film thickness. Too thin gives poor adhesion; too thick can crack, shrink, or trap solvent.
- Allow flash-off between coats. Solvent trapped under a topcoat causes solvent pop and blisters.
- Sand primer-surfacer with the recommended grit, then clean before basecoat. A sealer is usually reduced and applied as a thin coat; do not sand it unless the sheet allows.
- Prime bare metal immediately after preparation. Do not leave bare steel overnight in a damp shop.
- Use a contrasting primer colour to help coverage, but do not rely on colour to hide thin spots.
Worked example
A bare steel panel is cleaned, etched, and coated with a heavy single coat of primer-surfacer, then topcoated after 10 minutes. The finish blisters. Reasoning: the primer was applied too heavy and not given enough flash-off or cure; solvent remained trapped. Correct method: apply epoxy or etch primer as specified, then primer-surfacer in two or three thin coats with flash-off, cure, sand, clean, seal if required, and topcoat.
Traps
- Etch primer over galvanised steel or over cured paint without checking the system.
- Sanding sealer before it is ready.
- Using primer-surfacer as a sealer without reducing it.
- Applying topcoat over primer that has been sanded but not cleaned.