Garage heater affect painting?

Temperature and humidity have an inverse relationship.

High RH and low temperature becomes lower RH at a higher temperature

Here is Colorado we have to add moisture to our house ac systems to keep the RH at about 30%

Now if you are exhosting the products of combustion into you garage...

All bets are off.

I forgot to mention, some coatings are moisture cured, meaning if the RH is too low it will take forever to cure.