I use paper toweling over a CLEANED used filter if I am doing a vehicle with alot of pet hair in it.
Keeps the filter from getting clogged with hair.
You may need remove the hair from vacuum filter with the paper toweling and place new toweling on the filter,
depending how much hair up suck up to allow suffcient vacuum suction as you clean.
If you are using bags to minimize the need to clean your vaccum's air filter, I would venture to guess that you are loosing alot of the vacuum's suction efficiency.
I clean my filter quite often, but I not cleaning out 6 cars a day with it either. You can do a quick-clean of your filter media by removing it from your shop-vac and just tapping it inside a large garbage can to loosen the collected debris from the media. Common sense suggestion: Do it AWAY from where you work on cars. (Like not in the garage where your car is because loose ,fine dust ends up settling on your car's surface or outside and downwind from your car sitting in the driveway because wind-driven dust ends up on your car)