If you still have straight line scratches remaining, then you're not being aggressive enough with your defect removal process. You have stated that you are using SF4500 and a finishing pad, so you really can only get more aggressive as this is one of the least aggressive polishes on the market.
If your scratches are isolated to a small area, spot compound the defects and then polish afterwards to restore gloss. If the entire car is trashed and has moderate scratches (that are removable) then you will likely have to do a multi-step correction on the entire vehicle to see the results you are looking for.
In any case, here is how I would proceed.
- Begin your testing all over again
- Start with an area that has these scratches, and use a medium polish (if you're a menzerna fan, grab some MC 2500) on a polishing pad and see what you get
- If there was no noticeable improvement, move up to a cutting compound and a cutting pad. If there was an improvement, but you still faintly see the scratches, perhaps just use a more aggressive pad with the same medium polish and see if that produces a better result. If the scratches are gone, mission accomplished, you have removed the scratches with the least aggressive method.
- Once the scratches are removed, you will need to evaluate your finish. Since it has been noted that the paint you are working on is quite soft, there will likely be micro marring. Proceed by testing various finishing polishes & pads to determine the best combination to produce a clear finish.
Hope that helps
-Zach