Marine upholstery thread guide
Marine upholstery is sewn with UV-stabilised bonded polyester thread, most commonly Tex 90 for general seat work and Tex 135 where the seam takes real load. Bonded nylon is stronger and more abrasion resistant, but it degrades in sunlight and absorbs water, which is why polyester is the outdoor standard. Cotton and cotton-wrapped threads have no place on a boat at all.
This is the cheapest component in the job and the one that most often ends it. A seam that fails takes the whole cover with it, however good the vinyl was — and thread sits exposed on the surface of the seam, taking more direct sun than the material either side of it.
Polyester or nylon
| Bonded polyester | Bonded nylon | |
|---|---|---|
| UV resistance | Good — the reason it is the outdoor standard | Poor; degrades measurably with sun exposure |
| Moisture | Low absorption, dimensionally stable wet | Absorbs water, swells, loses strength wet |
| Raw strength | High | Higher |
| Abrasion | Good | Better |
| Right for a boat? | Yes | Only where it is never in sun or wet |
Nylon wins on paper and loses on a boat. Strength is not the property under attack — sunlight and water are, and those are precisely where nylon gives ground. A nylon seam on a sun-exposed seat can look sound and pull apart at the stitch line in a couple of seasons.
What Tex actually means
Tex is the weight in grams of 1,000 metres of thread, so a higher Tex is a heavier thread. It is a direct scale, unlike the older sizing systems, which is why spec sheets use it.
| Size | Use |
|---|---|
| Tex 70 | Lighter fabrics and decorative topstitching where load is low |
| Tex 90 | The general-purpose marine seat size — seams, panels, most cushion work |
| Tex 135 | Heavier seams, structural stitching, covers and enclosures, thick material stacks |
| Tex 210 and up | Heavy canvas and industrial work; usually more than seat upholstery needs |
Going heavier is not automatically better. Oversized thread in a light material makes large needle holes, and on a boat every needle hole is a path for water into the foam. Match the thread to the material and the load rather than defaulting to the heaviest spool on the shelf.
What "bonded" means and why it matters here
Bonded thread has a resin coating applied over the twisted filaments. It holds the plies together, cuts friction through the needle and the material, and stops the thread untwisting or fraying at speed.
On marine work the coating does something extra: it resists wicking. An unbonded thread can carry water along the seam by capillary action and deliver it straight to the foam, which is the same route that feeds mildew and pink staining. Bonded thread is standard for a reason.
Needle, tension and stitch
- Match needle to thread. Too small a needle shreds the thread; too large leaves holes bigger than the thread fills, and on marine work those holes take on water.
- Lockstitch, not chainstitch, for seat seams. A chainstitch can unravel from a single broken stitch; a lockstitch fails locally.
- Backstitch or lock the ends. The end of a seam is where failure starts.
- Keep stitch length sensible. Very short stitching perforates the material into a tear line — which is the same failure mode as a stamp edge.
- Consider seam placement. A seam sitting where water pools will fail before one that does not, no matter what thread is in it.
What we stock
| Product | Use |
|---|---|
| OutdoorPro Tex 90 — 1/2 lb spool | Mid-weight bonded polyester for marine, automotive and outdoor upholstery. UV-stabilised and water-repellent. The general-purpose choice for boat seats |
| OutdoorPro Tex 135 — 1 lb spool | Heavier bonded polyester for structural seams. This is the thread we run on every CNC sewn panel we produce |
That last line is not a marketing point so much as a disclosure: it is what we use on our own production work, which is the honest test of whether we believe in a thread.
The thread mistake that costs a whole cover
Recovering a boat in premium marine vinyl and sewing it with whatever thread was already on the machine. It is the single most common failure we see, and it is the cheapest one to avoid — thread is a few percent of the material cost of the job.
If a set of seats has failed at the seams while the vinyl either side still looks fine, the thread was wrong. That is worth knowing before the re-cover rather than after.
Choose OutdoorPro Tex 90 in a half-pound spool or Tex 135 in a one-pound spool. Check the thread size against your machine and needle before ordering. Call 336-887-3101 if you need help choosing for the job.