#4 Spline is a great all rounder for applying filler on intermediate surfaces. It's ideal for sanding transition curves between sweeps and a panels edge.
The term "Spline" is used to describe the construction of a smooth curve. Working from a few control points (high spots) the Spline will take the path with minimal overall bending - this being the final desired surface.
How does this relate to refinishing cars?
Almost no surface on a car is flat or straight; nearly every surface has a shape. Very rarely is any surface a true radius. Every surface was created in a design studio with a Spline, either on a clay model or a computer screen. The surfaces bend, sweep and accelerate and only Splines can recreate this form. Using these to repair a surface is the most accurate way.
We can get good results with traditional sanding blocks, although blocks constantly take the average of the immediate surface around it. Blocks do not take into consideration the overall form of the surface. Whereas, a Spline allows absolute control of the surface.
Splines are the most accurate tool for perfecting bodywork. Whether it be in metal, filler, blocking sanding, or in final sanding ready to polish paint. It is the user’s decision to perfect the surface in steel, filler, or paint depending on the level of adjustment. Splines are used to find highs and lows, panel alignment, wipe in filler and sand from 80grit to 800grit. Yes, one tool does all that
#4 Spline 500 x 70 x 2.4mm THK