Polyester Resin Unwaxed

SKU PR4

by Ark Composites

$89.00 - $289.00

$89.00

$89.00 - $289.00

$89.00

Quantity

Building up a hull repair, glassing a new deck or laminating stringers? Unwaxed polyester resin is the go-to fiberglass resin for marine layup. With unwaxed resin, the top layer of the resin itself acts as the air barrier. The resin underneath cures, while the top layer stays tacky (uncured). That tacky surface is what gives an excellent bond between layers, so it's the most popular choice for larger, multi-layer projects.

What makes it work

Layers lock together. The air-inhibited surface gives the best secondary bonding for layup after layup.
Just add catalyst. It's prepromoted and thixotropic, so it only needs MEK peroxide hardener.
Reasonable working time. It gels in about 20 minutes at 25 °C with 1.5% MEKP.
Cures fully, even thin. Medium reactivity cures thin laminates completely at the right catalyst level and temperature.
Tough results. With the right amount of glass it makes a tough laminate that resists cracking.

Good for

Fiberglass boat repair: hulls, decks, transoms and stringers
Laminating fiberglass mat and cloth in several layers
Shower pans, bathware and exterior wood decking
General fiberglass fabrication

Specs at a glance

Spec	Value
Type	Unwaxed (air-inhibited) general purpose polyester laminating resin
Gel time	20 min at 25 °C with 1.5% MEKP
Peak exotherm	208 °F / 98 °C
Viscosity at 25 °C	550 cps
Thixotropic index	3.5+
Styrene content	45.1%
MEKP catalyst level	1.0–2.0% of resin weight
Shop conditions	18–32 °C (65–90 °F), 40–90% relative humidity
Cured casting (no glass)
Tensile strength	12,000 psi / 84 MPa (ASTM D638)
Tensile modulus	560,000 psi / 3.9 GPa (ASTM D638)
Tensile elongation	3.6% (ASTM D638)
Flexural strength	20,800 psi / 143 MPa (ASTM D790)
Flexural modulus	580,000 psi / 4.0 GPa (ASTM D790)
Heat distortion temperature	153 °F / 67 °C at 264 psi (ASTM D648)
Sizes	1 gallon · 5 gallon

Typical values for unreinforced, post-cured resin castings tested at 25 °C (77 °F) and 50% RH; not specifications.

Tips for a good layup

Get the catalyst right. Use 1–2% MEKP by resin weight. A catalyst dispenser takes the guesswork out.
Watch the shop. Work between 18 °C and 32 °C and 40–90% humidity. That matters in an unheated garage in spring or fall.
Test first. Pigments, fillers, heat and humidity all change the gel time, so mix a small trial batch before the real one.
Keep going while it's tacky. If the layup has sat a long time, or been out in the sun, heat or dust, scuff it before the next layer. Always sand anything made with waxed resin before bonding to it.
Over gelcoat: wet the gelcoat with a thin coat of catalyzed resin first, then lay in the glass.

Unwaxed vs waxed

Use this unwaxed resin for the build-up layers. Since it stays tacky, finish the last coat with waxed polyester resin, or mix Styrene Wax Solution (Air Dry) into the final batch. Either way the wax rises to the surface and cures it hard and non-tacky, ready to sand or paint.

Before you use it on a boat

Below the waterline? If you want better resistance to osmotic blistering, put a vinyl ester skin coat down first.
No fuel tanks. Polyester resins like this one hold up very poorly against hydrocarbons and other hydrophobic liquids, so don't use it to build fuel tanks.

Common questions

My unwaxed resin is still sticky. Did it cure?

Yes, that's normal. Air stops the very top from curing so the next layer can bond to it. The resin underneath has cured.

What do I use for the final coat?

Waxed polyester resin, or unwaxed resin with Styrene Wax Solution (Air Dry) added. Both cure tack-free.

How much hardener per batch?

1–2% MEKP by weight of resin. About 1.5% gives a typical 20-minute gel at 25 °C. Do a small test batch first.

Can I use it for a fuel tank?

No. It has very poor resistance to fuels and other hydrocarbons.