For things dealing with higher pressure, I personally would recommend primer.
Higher pressure in our setups would be a closed loop (weight of the water of the entire tank pushing on the loop plumbing). A return is *high* pressure, but if it's not restricted (smaller output than the pump's output) it's not really that high of a pressure.
When it comes to Flex, I use Rain-R-Shine, and only use primer on the hard pvc fittings - the Flex just gets the glue (flex needs to be wiped clean with a clean rag first though)
....I just re-plumped the return to my closed loop this morning in fact, and waited a cool 10 minutes until the valves were opened again and the pressure applied to the new joints, no issues
