Subbed.
I'm always hesitant to offer advice, because of the amazing talent pool here on AutoGeek - but in my experiences, a mixture of allowing TRIX to dwell on the paint, along with an APC spray on severe areas being wiped with a properly saturated and softened bug-sponge that I drizzle with citrus-based gel cleaner ALWAYS takes off the hardest tar and contaminants.
I actually do a pressure-washer spray down and then do this step BEFORE the wash, as I've come to love the "prep" phase over the years as the MOST crucial step toward ensuring a positive outcome.
PS: Like Desertnate stated above, the wax coatings will be totally removed... but I actually ALWAYS strip client's vehicles anyway (unless, obviously, it is a repeat client that we service multiple times through the year and have had the advantage of proper detailing in the past)