I've used ITC-100 for years. I'll give you my advice/experience with it.....
I have never used it so I am not sure how it works.
ITC-100 serves a couple of advantageous purposes. First, it serves to "seal" ceramic fiber blanket materials, to MINIMIZE the fibers floating around, and to keep a person from breathing those fibers (very bad stuff for your lungs). Secondly, because ITC-100 contains ceramic, it will give you more heat reflection inside your forge versus most other products.
Do I paint it on the inside of the forge then lay my ceramic fiber insulation over it.
No. As Doug said, line your forge body with the ceramic fiber, then coat the exterior of the ceramic fiber (the exposed ceramic fiber that is visible inside the forge). Many people who do not have experience with ITC-100 go way overboard when applying it. Here's what I recommend: Put a few tablespoons of ITC into a small mixing cup, add very small amounts of water and mix until it's about the consistency of latex paint. Using a spray bottle, lightly wet the surface of the ceramic fiber (this is similar to using a primer for paint...without it the ITC-100 will likely not stick). Using a cheap paint brush, "paint" the thinned down ITC-100 onto the exposed ceramic fiber. It only takes a THIN coat... "more" is not better in this case, it just wastes ITC-100.
Here's where my advice differs a bit..... when it comes to drying/curing, I feel that it must be allowed to dry/cure naturally. If you get impatient and try to force dry it by firing the forge, all you're doing is shortening it's use life. Most of the time if you fire the forge, the ITC coating will start cracking. If this occurs, it will start falling off shortly there after. By nature we are not patient creatures, and it may take some time for it to cure naturally, but I can tell you that if you rush it, you're going to wreck it. For the environment where I live, it's not uncommon for it to take 2-3 weeks for a topcoat of ITC to cure naturally.... and the climate here is dry with very low humidity compared to many places. The only way I have found to aid in curing time versus wrecking it by firing the forge too soon, is to use a heat lamp/bulb, and just leave it on, inside the forge.
If I can save money if fuel cost I am going to do so
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Because of it's higher heat reflection, ITC will give you higher heat in the same forge, at the same fuel pressure, versus non-coated linings. That being said, you MIGHT get some fuel savings out of it, simply because it will allow to achieve a given temp with slightly lower psi of fuel....but it's not a huge savings....more of a long term savings then a "right now" sort of thing. For my purposes, the increased temps I can pull at the same fuel psi is what endeared me to ITC-100.
