Polymeric Sand for Flagstone and Natural Stone: Wide Joints

Flagstone and other natural stone rarely have neat, even joints. Gaps can run from a finger's width to several inches in the same patio, and the stones vary in thickness. That is why choosing polymeric sand for flagstone starts with the data sheet: you need a product the manufacturer rates for natural stone, for your widest joint, and for the base your stones sit on.

The short answer: several products we carry list natural stone and wide joints on their data sheets. They include Alliance Gator Dust Bond and SRW X-Treme (joints up to 6 in), Gator Maxx G2, SEK PolySweep X-Treme, Techniseal HP NextGel and Techniseal NOCO (up to 4 in), and Gator Super Sand G2 and Eurostone Bond for narrower stone joints. They differ on compaction, base type and temperature, so compare them below and test on your stone first.

Whatever you choose, follow the instructions on the bag and the manufacturer's current data sheet. Every figure here comes from those documents.

Why flagstone joints need a different approach

Three things make natural stone harder than concrete pavers:

• Wide, irregular joints. Techniseal's NOCO data sheet recommends flagstone joints up to 2 in with intersecting spaces up to 4 in, which shows how much widths vary on a single job.
• Compaction. Uneven stones can be hard to run a plate compactor over. HP NextGel's data sheet says to use a paver roller on natural stone, and if you use a plate compactor, to add a membrane such as flat-joint plastic snow fence or 5/8 in plywood.
• Porous surfaces. Porous stone can hold moisture and residue, so dry conditions and a thorough cleanup matter more.

Polymeric sand for flagstone: products rated for natural stone

This table lists what each data sheet (or, for Eurostone Bond, Alliance's product reference guide and data sheet coverage) states. Coverage is per 50 lb bag and depends on stone size, thickness and joint width, so treat it as the manufacturer's estimate.

Two notes on the numbers. SEK publishes conflicting coverage for PolySweep X-Treme in natural stone joints under 2 in, so we leave that band out; confirm with SEK or use our polymeric sand calculator for a project estimate. SRW's X-Treme data sheet also says it is not recommended for pavers, clay pavers, concrete overlays, pool copings, or wet or heavily shaded areas, and is for pedestrian use only.

For a side-by-side look at our sands, see the polymeric sand comparison page.

Stone dust style jointing: Gator Dust Bond

If you like the look of stone dust joints, Alliance Gator Dust Bond is a polymeric stone dust: polymer binders mixed with calibrated stone, in Sahara Beige and Stone Grey. Its data sheet lists these conditions:

• Joints 1/2 in to 6 in, pedestrian use only, for wet cast and natural stone. The sheet says it does not haze either.
• Install above 59°F, with no rain forecast for 24 hours; it is rain safe 24 hours after water activation.
• Traditional base only: a maximum of 1 in bedding sand over at least 6 in of crushed stone compacted to 98% Proctor density, or Gator Base panels. Do not use it when stones are set on a concrete base.
• Minimize walking on the stones for 24 hours after installation.

Match the product to your base

How your flagstone was set narrows the list quickly:

• Stones on compacted gravel and sand: every product in the table lists a draining or traditional base.
• Stones on a concrete slab or cement-sand bedding: Techniseal NOCO and HP NextGel list concrete overlays and cement-sand bedding, and Maxx G2 lists overlays with joints up to 2 in and drainage holes. PolySweep X-Treme lists concrete overlays with a 1 in bedding sand layer and drainage.
• Not for concrete: Gator Dust Bond and Super Sand G2 say not to use them over concrete. SRW X-Treme is not recommended for concrete overlays and is not a substitute for mortar.
• Pool areas: Alliance's Dust Bond data sheet and Eurostone Bond instructions say pool coping joints must use mortar.

Install tips for polymeric sand on flagstone

1. Start dry. Stone tops, sides and joints must be completely dry (Alliance, SEK, SRW, Techniseal). NOCO's sheet warns that on very porous stone, a damp surface can cause staining that is very hard to remove.
2. Sweep over short distances. SRW warns that moving sand over long distances can separate the polymers, SEK says not to push PolySweep long distances, and Techniseal says to sweep NOCO over short distances.
3. Fill to full depth and settle it. For Dust Bond, sweep it into natural stone joints, strike the stones with a rubber mallet, and repeat at least two more times.
4. Leave a recess. Finish at least 1/8 in below the top of the stone (Alliance, SEK, SRW, Techniseal).
5. Clear every grain off the surface with a soft broom and leaf blower before watering. Alliance warns Dust Bond left on the surface can activate from moisture and stain.
6. Water exactly as directed. Dust Bond: shower 50 sq. ft. for 90 seconds. NOCO on flagstone over a draining base: 50 sq. ft. for 45 to 60 seconds, stopping once the joint is activated to 75% or more of its depth. SRW X-Treme: 100 sq. ft. sections for a total of 2 minutes, then check for at least 1 in of moisture depth with a screwdriver.
7. Protect it while it cures. NOCO must be protected from rain for 24 hours (48 hours may be required for wider joints or colder weather) and needs temperatures above 50°F for 48 hours. SRW X-Treme is rain safe in 20 minutes but needs temperatures above 32°F through curing.

Test for staining, halo and haze first

Natural stone varies, so test before committing. Techniseal's HP NextGel and NOCO data sheets both say to test on a hidden area of about 4 sq. ft., then wait two days to confirm no halo forms around the stone edges. They explain that a temporary moisture halo is different from "picture framing," a permanent migration of jointing ingredients into porous stone that can occur with all jointing systems, including mortar and epoxy.

SRW says to test X-Treme in a small, discreet area. Order polymeric sand color samples to check color against your stone before buying full bags.

Frequently asked questions

What is the widest joint polymeric sand can fill on flagstone?

Among the products we carry, Gator Dust Bond and SRW X-Treme list joints up to 6 in. Maxx G2, PolySweep X-Treme, HP NextGel and NOCO list up to 4 in.

Can you use polymeric sand on flagstone set in concrete?

Some products allow it. NOCO and HP NextGel list concrete overlays and cement-sand bedding; Gator Dust Bond, Super Sand G2 and SRW X-Treme do not.

Do you need a plate compactor for flagstone?

Not always. NOCO needs no compaction, Dust Bond uses a rubber mallet, and PolySweep allows a rubber hammer and wood. HP NextGel requires mechanical compaction, with a paver roller on natural stone.

Will polymeric sand stain flagstone?

It can if applied to a damp surface or left on the stone. Techniseal also describes picture framing on porous stone. Test on a hidden area and wait two days.

How much polymeric sand for flagstone do you need?

It depends on the product and joint width. For example, Dust Bond lists 20 to 30 sq. ft. per 50 lb bag for 1 to 2 in joints, and NOCO lists 10 to 40 sq. ft. for wide flagstone joints. Check the table above and use our calculator for your layout.