I used the ZEP stripper, and it works great. Only 10 bucks a gallon. I used maybe a quart and a half to do the passenger side of my 33' motorhome. I use it with a 2 water to 1 stripper mix.....works fast.
That side gets brutal afternoon sun but the drivers side is in the shade and have only given it a new coat 2-3 times in the 8 years or so since I put on the ZEP. Have never stripped it.
When done with the stripper, I wash thoroughly with dish soap and rinse well.
Can't see putting 303 on first??? Seems like it wouldn't do anything for the ZEP. When I get a chance, gonna pick up some 303 and test it on one area.