Polyester resin and (I'm afraid) not enough hardener—problem?

I’m doing some wood finishing (on my drums, to be exact) with the polyester resin from a fiberglass kit I had lying around, you know, to make a hard reflective surface on the interior. It’s basically epoxy, from what I gather.

However, I’m afraid i didn’t use enough hardener—there was 1/4 bottle left, with I used with one-quarter of the bottle of resin per the instructions. I think I used enough, mind, but the label said that the resin would be dry in 40 minutes. It’s now almost an hour later, and it’s still tacky to the touch.

My question is, will the resin dry eventually in the case that not enough hardener was used? And, if it doesn’t dry and I have to remove this insipid blue goop, is something like paint thinner or lacquer thinner an effective removal agent?

It’s not an exact science, but generally, the more hardener you use, the quicker it sets, the less, slower.

Try to keep in the reccommended range, though. Too much hardener and it’s brittle. Way too little, and it never sets.

Give it another hour or so. If it’s still gooey, clean it out and redo.


be careful with lacquer thinner around plastic finishes. It can mar them unrepairably.

The can should have listed on it the reccommended solvent(s)

Well, I asked this elsewhere as well, and someone noted that the temperature and humidity comes into play as well…I’m sure you know the source of that one, NCB:wink:

They said they’d be a little worried if it were still tacky after 24 hours, so I’ll update then. Thanks for the reply!

Actually, they may just be wrong. It is not a water borne product. What hardens it is a chemical change caused by the catalyst/hardener.

This is what confuses a slew of people. They think that since it’s liquid-ish at some point that it behaves like a water borne/based product, like say… mud.

But even quick set drywall mud doesn’t dry, it sets. In that case, water is the catalyst.

Oh well… I know you’re smart. I just think certain people give uninformed advice.

Then again, I may be the uninformed doofus here, no? :slight_smile:

ftr, did you use the mesh? 'Cause just the goo might end up popping off when you actually use the drums.

Oxygen from the air acts as a terminator in the free radical based polymerization of polyester resins. If you don’t add enough hardener, the outer layer may not ever harden, because oxygen diffusing into the surface prevents the growing polyester chains from reaching a length at which they become solid. If the stuff still isn’t hard by morning, try setting it out in the sun for a few hours. The UV radiation will generate more free radicals, and may allow the reaction to go to a satisfactory completion. The one time I recall trying this, it worked. Of course YMMV.

No, I didn’t use the mesh; I just mixed up a 1/4 = 1/4 solution and applyed it with bits of foam rubber (with gloved hands) for want of a foam brush.

It’s a thin coating on top of 4 coats of Kilz, which I hoped would be enough to give a sufficiently hard reflective surface—alas, while it is stiff by itself, it wasn’t quite what I had in mind. I remembered Jaco’s bass fingerboard (epoxied for durability and action) and the fiberglass drum sounds of the 70s and figured a thin layer of essentially plastic glue would cheaply improve my toms, which are made of a cheap wood commonly found inside interior doors.

I did feel the resin an hour ago or so—I think it felt smooth in one place and a bit tacky in another (I don’t want to touch it too much), so I think it’s improving. In the case that they don’t dry by tomorrow, I will leave them in the sun for a few hours.

First off, polyester resin and epoxy are two completely different resins.

Optimun catalyst ratios (hardener is really a misnomer)can vary from 1/2 to 2 % of resin volume depending on temperature. There is a lot of leeway for most practical purposes with polyester catalysation ratios, wheras epoxy cures are optimized by specific ratios regardless of temperature. I n your case, the polyester cure was most likely delayed due to a lower temperature than the usual 68 degree. The other possibility is that the resin was laid on too thin, and the required exotherm to complete the cure was ineffective.

Look, forget dry. did the resin harden? If so good. The surface (top several molecular layers) of a polyester cure is always tacky because air is an inhibitor of polyester polymerization. Had you added “air dry”, a paraffin in styrene solution to the resin, you would not have experienced the tackiness.

Both polyester and epoxy do not stand up well to UV. If sunlight is not an issue, then I suggest another coat of poyester resin modified with “airdry”. If your work is subject to sunlight, then I suggest a clear gelcoat that includes airdry or waxed.

The other alternative to remove the stickiness is to wipe the surface with acetone soaked paper towels. This could however get messy.

I purchased some fiberglass resin. It came with some catalyst but I lost the catalyst. What can I use as a substitute? Any household goods? I have the catalyst from fiberglass filler and the catalyst from body filler, both used in repairing vehicles. Can I use either one of these catalysts as a substitute?


I’m sorry, but there is no such thing as “fiberglass resin.” You can use fiberglass with lots of different resins, however, polyester resin is far and away the most common resin used with fiberglass.

That having been said, the most common catalyst for polyester resin is Methyl Ethyl Ketone Peroxide, so check the contents of what you have for MEK-P.

Any way you slice it, you should definitely try a test with a small amount of resin first. I wouldn’t worry about ratios too much for the test, I’d just put a drop of catalyst on a small dollop of resin, mix and see if the resin hardens.

13 year old thread.