Filler

I need to lay some filler on the weld joins in places, wont need much so shouldnt take to long but the question is:

Will it be ok to lay filler with the weather being below freezing or will it make it crack and fall out? either straight away or in a few months once its all painted?

 

I would gently warm with a heat gun a little before and after
until gone off

 

With the weather the way it is at the moment, condensation is the enemy, if you can get the van into a workshop with a bit of heat you'll be fine, use a bit more hardener, and/or a heatgun/heater to help the filler go off.