All about polyester finishes

Cure times and application techniques for polyester finishes. January 31, 2001

I have several questions about polyester finish.

If it has a pot life of 15 - 20 minutes, how long does it take to harden?

Is it a water white finish?

How many coats can you put on in a day, and how many coats total?

What is the difference between polyester resin used in fiberglass work and polyester finish, other than the resin being thicker?

Forum Responses
The gloss topcoat in the ICA product line takes 10 hours to dry completely. It is not water white, but very clear. The product is touch-dry in 2 1/2 hours, at which point you can sand and recoat, with a total of 8 to 10 mils. The polyester resin for finishing wood is ground very fine so that there is good clarity. Fiberglass products don't need that.

It will harden in your gun in 10 to 15 minutes. I normally use Duratec, from Hawkeye and Ramanel from Lilly. Duratec cures to a rub-able finish in about 10 hours. Poly dries from the bottom up, so it is buff-able even if the topcoat feels a little tacky.

Once you have fogged on your tack coat, you can lay on as much as 40 mils in repetitive coats. If you treat the product like lacquer and try to spray on nice light flow coats, you will get into trouble. Spray on until you think you have applied too much, then lay on two more coats, and it will flow out.

I have never used it in a situation were I needed a true water white finish.

With Ilva polyester, hardening time can be shortened or lengthened by using different levels of accelerator and different catalysts. The range of hardening is 30 - 45 minutes touch dry to 8 - 12 hours total cure or 45 minutes - 1 1/2 hours touch dry to 12 - 16 hours total cure.

I don't know of any polyester that is water white, but Ilva polyester has excellent clarity. I have put on 6 coats at approximately 15 mils each with 30 minute tack times in between each coat. I find clear fiberglass resin, as in boat type, to be more brittle and not as clear as a furniture-type polyester. The polyester that I am referring to is great for clear coats over furniture. We use polyester for the build coats and the top coats if they are high gloss. If a sheen is needed, we use polyurethane top coats that are water white.

From the original questioner:
Does anyone know the amount of time it takes for a full cure, where there is no more shrinkage?

By the next day it is basically done shrinking, but this does vary from brand to brand.

Bob Niemeyer, forum technical advisor

We are also using the ILVA polyester. We allow ours to dry about 4 days. The amount of thinner added to the polyester and the thickness of the coating will change this. The polyester resin itself cures very fast. It is the solvents added to the coating that cause it to shrink.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
I use different types of polyester from Sayerlack. They are either buffable or direct gloss. To save the buffing time I have used direct gloss PL1243 from Sayerlack. I have put around 300 microns at a time and given around 30 minutes between coats. I have done altogether 3 coats. PL1243 gives clarity very near to the water finish you are referring to.