Build a Radius Block That Repeats a Handrail Profile

A repeatable handrail sanding block needs more than a vaguely curved face. Build its concavity from the rail's actual cross-section, then verify contact at the crown and both shoulders with the abrasive installed. That turns the block into a registration tool instead of a pressure point that happens to fit in the middle.

This method is for a repeated, stable handrail profile. Do not assume an old rail is circular, symmetrical, or identical from end to end. Carved details, a changing profile, loose joints, lead-containing coatings, or a one-off repair call for a different plan.

Capture the profile you need to repeat

The best reference is a clean offcut from the same rail stock. Stand the offcut against the end of the block blank and trace the upper arc plus the shoulder transition. If the profile is painted, damaged, or hand-shaped, use a representative sound section rather than copying a dent or paint buildup.

For a truly circular crown when no offcut exists, first make an accurate cross-section tracing from an accessible square end face. If there is no accessible end, use a non-marring profile-transfer method suitable for the existing finish, then check the transferred outline back against the rail. Work on that two-dimensional tracing instead of trying to bridge a straight strip through the solid crown:

1. Mark two repeatable shoulder points on the tracing and draw the straight chord between them.
2. Measure the chord, then measure the perpendicular rise from its midpoint to the traced crown.
3. With both dimensions in the same units, calculate the radius as R = c² ÷ (8s) + s ÷ 2, where c is chord length and s is rise.
4. Set a compass to that radius and strike the concave cutline on the end of the block blank.

This calculation applies only to a circular arc. If the left and right halves do not mirror, or the crown blends into beads and flats, transfer the complete cross-section instead. Label the rail location and orientation on the template so future blocks repeat the same reference.

Cut a registration face, not a tunnel

Choose a straight, stable block blank wide enough to register on the intended shoulders without touching adjacent beads or flats. Transfer the profile to both ends and connect matching points along the sides. Remove the waste while staying outside the line, then refine the concavity until the reference just fits.

Keep the working face parallel along the block's length. A hollow that twists from one end to the other will steer the block across the rail. Lightly relieve the leading and trailing lips so the abrasive does not enter on a knife edge, but preserve the full central registration surface. Round the hand-grip edges separately; do not roll the working shoulders away.

The block should also be short enough not to bridge a planned change in the rail's path. Its cross-section controls the rail profile; it is not meant to straighten a curved staircase rail along its length.

Prove contact twice

First test the bare block on the offcut or a sacrificial profile. Put a sparse soft-pencil witness pattern across the sample's crown and shoulder zones. Seat the block without squeezing or rocking it, make one short axial stroke, and lift it straight away.

Read the remaining marks:

• loss only at the crown means a circular hollow is too open (its radius is too large) or the shoulders are not registering;
• loss at one shoulder means the block is skewed or the transferred profile is asymmetric;
• untouched crown with rubbed shoulders means a circular hollow is too tight (its radius is too small);
• light, distributed change across the intended arc means the face is close enough for an abrasive trial.

Correct only the high-contact areas inside the block, then repeat. Next fit the actual abrasive tightly, with no wrinkle or overlapping seam, and run the witness test again. Paper thickness reduces the effective radius of a circular hollow, so a bare block that fits can become shoulder-heavy once wrapped. This second test is the repeatability check that a generic “match the curve” instruction misses.

Mirka's profile-sanding guidance treats concave and convex shapes as distinct contact problems, while Norton's hand-sanding guide notes that a block distributes pressure on flat work. Here, the transferred concavity supplies that controlled support for one specific rail section.

Put the block into the sanding sequence

Use the block only after the witness pattern shows broad contact. Work along the rail axis with short overlapping strokes and no deliberate side-to-side roll. Stop after a small section, clean it, and compare the scratch pattern at the crown and both shoulders. A bright untouched stripe calls for a fit correction; extra hand pressure is not a substitute.

Where the specified finish schedule uses these grades, eQualle 120-grit sheets in a 10-sheet pack and eQualle 220-grit sheets in a 10-sheet pack are cut-to-fit options. Select only the grade the current step requires; the two links do not imply a direct jump.

For surfaces that should flexibly follow an existing contour instead of registering a repeated radius, compare a firm block with a conformable sponge.

Short tip: following a handrail contour

Store the finished block with its template, rail location, orientation, and abrasive grade range marked on it. Before the next use, confirm that the face is still clean and true; a trapped chip can turn a repeatable support into a repeatable scratch.