What time of year is best for undercoating?

Best time of year for undercoating depends on whether you mean house painting or car rust protection. For house painting, best time to apply undercoat and paint interior is any time with controlled indoor conditions, but fall and winter when humidity is lower and painters less busy is good for interior. For exterior house undercoat and paint, best time is late spring through early fall when temperatures are 50 to 85 degrees Fahrenheit and low humidity and no rain expected. For car undercoating for rust protection, best time is late summer to early fall, August to October, before winter salt season starts, so coating has time to cure and provides protection when roads get salted. You should apply car undercoating when underbody is dry, clean, and warm, not in freezing winter or wet spring when moisture trapped. Applying in fall gives maximum protection during harsh winter. If you live in area without snow, any dry time is fine. For oil based annual rust proofing, fall is ideal before first salt. Avoid applying car undercoating in very cold, wet, or humid conditions as it will not adhere well. So for house exterior, late spring and early fall, for car rust protection, late summer to early fall before winter is best.