I was prepping the surface using PoliPrep yesterday and I was going to post today how I didn't think it did a very good job. However I went outside this morning and it is clear now that it's dried how well it worked to take out most stains. There is some black gunk (algae perhaps) that I just can't seem to get off with anything but it's minor and I can live with it.
Saying that, I am considering not using the PoliGlow that I bought but instead using Penetrol
which is what a few neighbours have recommended - they use it on their fiberglass campers.