Unpopular opinion: most "deep scratches" I see don't need filler at all
A guy rolled into my buddy's lot off Route 9 in Jersey last month with a rear quarter scrape he swore needed a full repaint, and I told him to hold up while I hit it with 1500 grit and a DA. Turned out the clear was just scuffed, not cut through, and 20 minutes of wet sanding and a polish had it looking close to new. He paid us $150 and left happy instead of the $700 he walked in expecting. Not saying filler never has a place, but half the cars I look at get sanded down for nothing. How do you all decide when to stop sanding and actually reach for the mud?
Man, the thing nobody talks about is how the lighting in the shop messes with your head on this call. Under a sodium bay light or a cloudy day, a scuff looks like bare metal, but roll it into direct sun and you can see the clear is still sitting there just hazy. I keep a little LED pen light in my pocket for exactly this, I hit the scratch at a low angle and if the light stops at the edge instead of running down into it, that tells me whether I'm into primer or not. Saved me from grabbing the mud more times than I can count, and it costs like ten bucks.