What to avoid when undercoating?

What to avoid when undercoating includes mistakes that cause poor adhesion, trapped moisture, and premature failure, for both house and car. For house painting, avoid undercoating over dirty, greasy, dusty, or wet surfaces, as paint will not stick, always clean and let dry. Avoid undercoating over peeling, flaking, or chalky old paint without scraping and sanding first, as new coats will peel. Avoid using wrong type of undercoat, such as interior primer outside or water based primer over tannin rich wood without stain blocking, which causes bleed through. Avoid applying too thick or too thin, follow spread rate. Avoid painting in extreme temperatures or high humidity. For car undercoating, avoid undercoating over existing rust without removing loose rust and treating with rust converter, as coating will trap moisture and accelerate corrosion. Avoid spraying undercoating on exhaust system, brakes, drive shafts, engine components, and rubber bushings, as it can cause fire, brake failure, or damage rubber. Avoid applying in cold wet conditions. Avoid cheap used motor oil as undercoat, as it damages rubber and is environmentally harmful. Avoid applying too thick that it cracks. So avoid poor prep, wrong product, coating wrong parts, and wet rusty surfaces.