I have sprayed thousands of gallons of US paint over the last 25 + yrs,,,I'm gonna take a guess and assume you talked to a fon rep that was reading from a tech manual with no application experience,,,especially after they recommended "watertight" which was designed as a filler for Interprotec barrier coats (bottom of hull,under water surfaces) and sands like concrete,,,Give me a call and I will happily walk you thru the process and save you some time and a lot of aggravation...Yes you can use polyester fillers or vinyl ester fillers over 545 for "small" spot repairs and pin hole filling and yes It Will Stick if done correctly,,,I have Awlgrip jobs that are pushing 20 yrs old with poly fillers on the 545 which have never lifted,,,But yes you are better off using an epoxy based filler like awlfair (but leaves a lot of pin holes) or better yet Interlux's interfill fairing compounds...Axzo Noble which is the parent company for Interlux bought out US Paint a bit over 10 yrs ago,so that is why they are recommending interlux products ,which in turn won't void the warranty,,,but warranty will only cover product replacement an Not Labor,,,And if you're not a Pro good luck gettin any money outta them...

Give me a call and I will happily walk you thru the process
Pete
401-252-9246