I cannot comment on the interior products as I have not used them.
It states on the Ultima page that:
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Ultima Paint Prep Plus is a fine, non-abrasive polish that deep cleans paint, smoothes away swirl marks, water spots and fine surface scratches, and increases over-all paint gloss. As it cleans and polishes, Ultima Paint Prep Plus leaves behind a special primer designed to promote proper bonding of Ultima Paint Guard Plus paint protection. Using Ultima Paint Polish Plus greatly enhances and extends the life of Ultima Paint Guard Plus.
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So no you do not need it for it to work but to get optimum results you should use it. It is a nice product and has surprisingly strong defect removal capabilities. I used it again last night and had better results than previously. Remember with it though less is more, do not let it set as it is a bear to get off.