I am a valeter for a car dealership and we often repair scratches using this method-
Using this method I have this week repaired a 2 inch scratch on a brand new MG ZS in metallic Ignition Blue in less than 20 mins.
Obtain a touch up kit from Fiat
Fill scratch with touch up paint until slightly proud of surface (may take 3 to 4 coats depending on the depth) either dry by leaving a couple of days or a hot air gun (dont blister the paintwork by holding it too close!)
Use 1200 grit wet and dry with water and soap to gently flatten the paint concentrating mainly on the new paint keep drying the area with a tissue or kitchen roll you should see the new paint stand out from the existing paint keep flating the paint untill the blob disappears and turns the surrounding paint a matt finish.
Apply lacquer to the area where touch up paint was applied leave for 2 days or dry with hot air gun as above,
Flat area with 1200 grit paper as above then use 2000 grit paper water and soap to obtain flat a matt finish.
Polish using circular motion with G3 compound (Dont use T cut as butter is rougher!!) until a shinny finish is obtained.
Apply a good wax then your scratch should be invisible.
It takes a lot of work but its cheaper than respraying the panels.
Please make sure you are careful using the sand paper as it easy to sand through your paint work try practicing on an iconspicuous area first.
Hope this helps.