How much cut does it have?
Critical Finish has minimal cut and is mainly used for refining a finish to produce ultimate gloss at the end of a polishing programme; however, it does have an ultra-fine/fine amount of cut and will remover minor swirls, clay marring etc with the correct techniques. It is perhaps more suited to softer paint types (Japanese cars, custom or classic paint etc). For heavier cut use Lime Prime (fine cut) or Depth Charge (medium cut).
Can I use it by hand?
Yes, but machine application always gives the best results.
Will it fill defects?
Not much, it refines and enhances by removing and smoothing rather than filling. Always use a panelwipe like Dodo Juice Stripp-Ex after polishing to see the 'true finish' achieved.
When should I use it?
After heavier polishing stages to refine further/increase gloss, or for swirl or mark removal on soft or delicate paint finishes. Being ultra-fine it is the safest polish to go for, if unsure what polish type to use on your paint. This is why it is found in certain bundles/kits.