Silmar 249A vs 249B: One Additive Apart

Silmar 249A and 249B are the same resin with one additive between them. The A is the original water-clear, UV-stabilized polyester laminating resin. The B is that exact resin with an optical brightener added, a blue tint in the can that makes a white polyurethane blank read brighter white under the glass. That's the whole difference. Same base chemistry, same catalyst schedule, same cure, same lamination behavior.

First, the misconception to kill: 249B is not the sanding or hotcoat version of 249A. Both A and B are unwaxed laminating resins, and both cure with a tacky surface that needs wax on the final coat. The factory-waxed sanding resin in this family is Silmar 250, not 249B. If you bought 249B expecting a tack-free sandable finish, that's why your hotcoat never stopped being sticky.

We stock the original A formulation: Silmar 249A clear polyester surfboard resin in quarts, gallons, and 5-gallon kits, MEKP included. Glassing for paying customers? Apply for a wholesale account here.

The full Silmar surfboard family

Interplastic builds the whole line off one base resin. Knowing the map keeps you from buying the wrong can:

The pattern: A grades are neutral clear, B grades are brightened, 250 is waxed, LH is the air-quality variant, 2880 is the upgraded chemistry.

What the brightener actually does

Despite what some product listings imply, 249B does not dry white. It cures just as transparent as 249A. An optical brightener doesn't dye or pigment the laminate; it absorbs UV and re-emits it as visible blue-white light, the same trick laundry detergent uses to make whites look whiter. Over a white polyurethane blank, that fluorescence makes the board read as a crisper, brighter, factory-white and helps mask the slow yellowing every poly board eventually picks up. The white you're seeing is the blank through clear glass, boosted. In the bucket 249B looks distinctly blue; if it actually cured opaque white, nobody could cut a lap with it.

That same additive is why you don't want the B everywhere. Over anything that isn't white, the blue-white cast works against you.

Which one to buy

249A when the color underneath matters: tint and pigment work, resin swirls, wood veneer, carbon, repairs on a board that was glassed clear. The A is optically neutral, so your tint cures the color you mixed. Brightener under a tint shifts the hue cold, and on a repair over non-brightened glass, a B patch can flash blue-white against the surrounding laminate in sunlight. If you're working on an epoxy board instead, see our guide on how to fix an EPS epoxy surfboard.

249B when you're glassing a plain white poly blank and want it as white as possible, which is why production shops standardized on it for white shortboards. If you want maximum bright, that's what the double-brightened 249BB is for.

Handling is identical either way: kick with MEKP at 1-2% by volume (1% gives roughly 20 minutes at 75°F / 24°C), laminate wet-on-tacky, and put surfacing wax in the final coat, or hotcoat with 250, so it cures tack-free and sands clean. If you're ever weighing whether to use polyester here or switch systems entirely, our epoxy vs polyester resin comparison covers the tradeoffs in depth.

Bottom line

Pick by what's under the glass: 249A for clear, tinted, or repair work where neutral clarity wins; 249B for bright white boards over white foam. Everything else about the two cans is the same resin. We carry the A: Silmar 249A with MEKP hardener ships in quart, gallon, and 5-gallon sizes here. Doing this work professionally? Apply for a wholesale account here.