Polyester Clear Coat (Promoted) - What to do when I used too much hardener?

[Paul Rhoney]

I'm trying a new product, it's a polyester clear coat. The label also says "promoted" in parentheses after clear coat, but to be honest I have no idea what that means. I've heard there's polyesters that are three-part products, but this one is only two, the resin and the hardener.

The can label says to mix 2oz. to the resin, and it's a gallon can, so by my calculations that's a 64:1 mix ratio. Please correct me if I'm wrong. I'll never use an entire gallon of this stuff fast enough to just dump the little 2oz. bottle into there, so I have to be able to know the right mix ratio at much smaller volumes.

OK, so where I went wrong was I read those instructions, saw that the manufacturer's website says that they only ship the exact amount you need in the kit, but I had two bottles of hardener (4oz.), so I assumed they must have meant the resin portion was actually half a gallon, so I mixed it at a 32:1 ratio. After spraying it I thought to check the shipping invoice and saw where they noted to send extra hardener, explaining why I had two bottles, and why the can said 2oz. instead of 4oz.. This morning it's still not hard. It's sticky, not tacky like you touch it and it is all gooey onto your finger, but if I were to try and sand it that just wouldn't work.

What do I do now? I've read that I can just wait it out and eventually it will harden, but it will be brittle. Is that really a problem? A significant amount of it is going to be sanded off anyway, so why is brittle a problem? Another option I've heard is to put it out in the sun. Supposedly the UV rays and heat loosen the activators allowing them to do their job. Anybody have any idea?

[Paul Rhoney]

I should have thought to do this before posting, but I called the guy that suggested I use a polyester in the first place, and he answered my question. Apparently too much hardener isn't really a problem at all, it was just my inexperience causing the concern. A polyester coating needs at least 24 hours at 70° to fully harden. I stuck a heater in the room, and I'll check back on it later this evening.

[David King]

Keep it out of the sun. Don't try to warm it more than a few degrees or you may generate bubbles under the surface.

[Mario Proulx]

What sealer did you use under it? If you shot the polyester right over an exotic wood, it won't cure...

[David King]

Promoted vs unpromoted refers to the addition of cobalt or equivalent to speed up cross linking. Promoted resins have a finite shelf life.

[Paul Rhoney]

Thanks for the information David, that clears some things up.

Mario, can you elaborate a bit on that? I bought this can of polyester for use as a sealer, as in I asked my supplier for a can of "polyester sealer" and this was the stuff they sent me. The directions say that it "...has excellent filling properties and adhesion over fiberglass, metal, plastic and wood." So yes, I applied it right to raw wood (Honduran Mahogany and Spanish Cedar), expecting no issues that way. I'm not doubting what you're saying, I know you speak from experience. But of course now I'm worried that I'm screwed, and if that's the case I'd love to really understand why polyester right over the wood won't cure.

[David King]

Paul,
I can't imagine that the Honduran mahogany would give you any trouble but the Spanish cedar may have pitch pockets.
Oxygen in air will inhibit surface cure of many polyester resins. "Surfacing resins" contain 8% wax to allow a non tacky surface. Apparently you can also wrap the surface in saran wrap to keep oxygen away, I bet that leaves an interesting surface...

[Paul Rhoney]

Well I do suppose it should be noted then that the wood was grain-filled before application of the polyester. While that's not to say I had a perfectly level surface, I can say that the deeper pores were, you know, not deep anymore.

[David King]

Paul, What's the current state of this coat? Is it tacky but hard underneath? If it is have you tried wiping down the tackiness with acetone? Can you scrape the tacky layer off and get down to the hard surface underneath?
The woods that will directly interfere with normal hardening are oily rosewoods especially cocobolo.
My sense is that you needed more hardener for such a small quantity or that they sold you a "laminating" resin.

[Mario Proulx]

You're most likely okay with the mahogany. All rosewoods, ebony and a few others won't let polyester cure without a isolator coat. Sometimes called a barrier coat, sometimes simply called "rosewood sealer". Most of us just shoot the barrier over the entire instrument, then go to the polyester top coats.

Every polyester I've seen and used called for 2% catalyst. That works out to be 0.6cc MEKP to one ounce polyester. 1.2cc/2oz, 1.8cc/3oz, etc....

[Paul Rhoney]

I'm not at my shop today, but my shop mate is, and he says it seems like it's probably hard enough to sand, but I don't believe he's tried to do so yet.

Thanks for the explanation Mario. I actually knew about the isolator coats and I believe we asked the finishing supplier about it, but never got any. That slipped my mind when I was applying the polyester.

[Paul Rhoney]

So where it is right now; I sanded the initial coat down quite a bit, even burned through in a few places (this is a solid color finish guitar, so I'm not worried about witness lines under the color). I still had quite a bit of open grain, and other issues because as I mentioned earlier, the coat didn't flow in much at all, because I didn't have any acetone to thin it when I sprayed it. Next I put a little grain filler in the deepest unfilled areas, and I shot two more coats of polyester. This time I tried to mix it as close to 64:1 as I could, and I used a healthy amount of acetone to thin each coat. The coats were applied about an hour apart, and I did get some minor runs. With the temperature up considerably from where it was with the first coat I noted that it felt harder within a matter of hours compared to how hard it felt before after almost 24 hours.

Now I haven't been back to the shop since then, but my shop mate was eager to see how things were going and told me that he sanded the whole thing out the next day. He was very impressed with how well it builds, how every pore was filled, how easily it sands, and how it doesn't clog even fine discs up to 800 grit. He said it's ready for the color coat, and the only concern is maybe how thick it is.

Now my thoughts are that I probably could have got away with one less coat if I had thinned the first coat to begin with. I was easily able to see that the properly thinned coats that had a good chance to flow out did so with almost no orange peel. None of this is to say that I wouldn't have had better results if I had also used an isolant, so I need to get some of that stuff.

My plan at this point is to go ahead and shoot the color on it, but first I am going to peel the tape off of the fretboard and see how easily I can peel the polyester off of the edges of the fretboard. Assuming that doesn't end up a catastrophe I'll shoot color and see how easily I can peel the color off of the polyester (i.e., looking for adhesion issues) before I put clear coat on it.