Joint filler on a polished floor is either invisible, a deliberate line, or a defect. Which one you get is decided before the filler goes in: by the aggregate colour, the gloss level and the way the joint is cut.

Two valid strategies

Blend in. Match the filler to the dominant aggregate or the surrounding paste so the joint disappears at normal viewing distance. This suits monolithic floors and open-plan retail where the joint line would break up the surface.

Make it a line. Deliberately choose a contrasting tone, usually one or two steps darker, so joints read as an intentional grid. This suits large floors with a strong pattern and industrial looks.

How to match properly

1. Photograph the dry, finished floor next to a grey card in daylight, not with flash.
2. Take three candidate filler tones: one at the paste colour, one at the dominant aggregate, one half a step darker.
3. Fill three 300 mm test joints in less conspicuous areas and polish them with the same sequence as the floor.
4. Read them under the client's viewing light and at the client's viewing angle, then choose.
5. Record the batch numbers; a re-order from a different batch can shift a full shade.

Colour shifts after polishing

Issue	Cause	Fix
Filler turns darker than sample	Resin pads load with filler dust	Clean pads between joints, polish joints last
Chalky white joints	Filler not fully cured before polishing	Wait full cure time, usually 24 h
Gloss mismatch	Filler harder than surrounding paste	Choose a filler matched to slab hardness, not the cheapest
Visible halo	Filler smeared onto the floor	Mask edges, clean immediately with the correct solvent

Application detail that decides the result

• Joint width 3-6 mm; wider joints need a backer rod and a different product.
• Clean the joint dry, then prime if the manufacturer requires it.
• Fill slightly proud, cure, then shave flush with a razor blade before polishing.
• Never polish a fresh joint with the same pad you just used on the floor; the filler loads the pad and smears.

Match to the gloss level, not just the colour

A filler that looks perfect at 400 grit can read as a pale stripe at 2000, because gloss changes how the eye reads value. Two surfaces with the same colour but different reflectivity look like two different greys, and the joint is the place where that shows first.

Finish level	Filler that reads neutral	What fails
Satin, 400-800	One step lighter than the slab colour	Dark filler, which reads as a black line
Gloss, 1500	Exact match, or one step darker	Light filler, which reads as a chalk line
High gloss, 2000+	Exact match with matched sheen	Matte filler, which reads dull against the polish
Salt and pepper, dark	Filler close to the matrix, not the aggregate	Matching the aggregate, which looks speckled

Test the match after the last polish step, not before. Apply three candidate mixes to a 300 mm strip across a joint, polish them with the same sequence as the floor, clean and look under the lighting the client will use. Colour judged at 200 grit is a guess; colour judged at finish is a decision.

Filling sequence and cure before polishing

• Cut the joint clean and dry, to a consistent width. A joint that varies from 3 to 8 mm will show a filler line of varying width no matter how good the mix is.
• Prime the joint walls if the product requires it. Unprimed filler bonds to dust, not to concrete, and pops out under traffic within a season.
• Overfill slightly rather than fill flush. Filler shrinks; a flush fill becomes a shallow groove that collects dirt and reads as a line.
• Let it cure for the full window in the data sheet, not the window you have. Polishing semi-cured filler tears it and leaves a ragged edge.
• Cut back with the same grit you will polish from, in both directions along the joint, so the filler and the slab are cut by the same tool at the same time.

Where the joint is a movement joint, do not fill it solid. A rigid filler in a working joint either cracks or pulls the joint edges with it; use a semi-rigid or elastomeric product and keep the line deliberate.

Documentation that prevents a callback

Joint work is the part of a polished floor most likely to be questioned a year later, and the answer is almost always a photograph plus a product code. Keep three records for every joint type on the job.

• A close-up photo of the joint at handover, taken square to the line with a colour reference card in frame.
• The product name, batch and mix ratio used, including any tint added on site.
• The finish grit the joint was polished to, which is the number that explains any later change in appearance.

When a client reports that "the lines have gone lighter", compare the handover photo with the same view under the same light before quoting any work. More often than not the joint has not changed; the floor around it has darkened with traffic film, and a deep clean restores the relationship between the two.

On restoration work, clients notice joints more than they notice gloss. Spending an hour on three test joints prevents the callback that costs a day.