The best time to think about undersealing is before the underside has developed serious corrosion. In the UK that often means inspecting and protecting a suitable vehicle before prolonged winter exposure.
Why dry conditions matter
Preparation and product instructions matter. The underside needs to be in an appropriate condition for treatment, and the vehicle may need to remain parked afterwards while the coating settles or cures according to the product guidance.
Do you have to wait for summer?
No. A mobile treatment can be planned throughout the year when the vehicle, site and weather are suitable. If conditions are poor, the sensible option is to rearrange rather than compromise the job.
What if the car is already rusty?
Surface corrosion is different from heavy or structural corrosion. The vehicle should be assessed first so the preparation and treatment are appropriate.