It's easy to treat hardener speed as an afterthought once you've settled on a resin, but it's one of the decisions that most directly affects how a job actually goes. Get it wrong and you're either racing a batch that's kicking off before you've finished laying it up, or standing around waiting for a small repair to go off. Get it right and the whole job feels easier than it should.
What Hardener Speed Actually Controls
Both MAS Low Vis Resin and MAS FLAG Thick Resin are available with fast, medium or slow hardener, mixed at the same 2:1 ratio regardless of which speed you choose. What changes is your working time — how long the mixed epoxy stays workable before it starts to go off — and, connected to that, how quickly the job cures enough to move on to the next stage.
Slow: For Big Jobs and Warm Conditions
Slow hardener gives you the longest working time, which is what you want for large laminating jobs where you need to mix a big batch and have time to wet out and lay up multiple pieces of cloth before it starts to set. It's also the better choice in warm weather, since heat speeds up the cure regardless of which hardener you've chosen — a slow hardener on a hot day can behave like a medium hardener would in a cooler workshop.
Fast: For Small Repairs and Cold Workshops
Fast hardener is suited to small repairs where you don't need much working time, and to cooler conditions where a slower hardener might take longer than you'd like to cure. If you're working outdoors in a UK winter or in an unheated workshop, a fast hardener helps counteract the way cold temperatures slow epoxy cure right down.
Medium: The Middle Ground
Medium hardener sits between the two, giving a working time and cure speed suited to moderate-sized jobs in average workshop conditions — not so fast you're rushed on anything beyond a small patch, not so slow you're waiting around on an afternoon job. It's also the hardener MAS recommends pairing with FLAG resin when you're bonding, filling or filleting with Colloidal Silica Filler.
Matching Speed to Conditions, Not Just Job Size
The most common mistake is choosing hardener speed purely on job size and ignoring temperature. A big lay-up on a cold day might still need a slow hardener for working time, but expect the cure itself to take noticeably longer than the same job in summer. Conversely, a quick repair on a hot day can kick off faster than you'd expect even with a fast hardener. Read the room, not just the job sheet: check the workshop temperature and adjust your expectations for both working time and cure time accordingly.
If you're not sure which speed suits a specific job, our contact page is the quickest way to get a straight answer from the workshop team before you mix a batch you can't undo.
