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Greenpatch-421 A 53% alumina, wet, air setting thick patching mortar. This product comes pre-mixed and can easily be thinned with water.
Greenpatch 421 is a product made for repair of forges and foundries and is great for sealing Inswool/Kaowool insulating blanket.
I have personally used this product for sealing Inswool on the forges I make. It is my opinion, that this is a superior product to Satanite in this application.
Forge repair. It is rated to patch/repair existing forges or foundries in up to 3 inch thicknesses. Curing is recommended and more than one layer of this product is recommended.
Manufactured by Harbison Walker
Applying Greenpatch 421
Below is my informal but effective instructions on how I apply Greenpatch 421 to a forge. I will be making a YouTube video soon to illustrate this simple process.
I get a 2" wide, cheap paint brush and cut off part of the handle so you can get it inside the forge. You can apply it with a paint brush. If it tears the inswool/kaowool then thin the Greenpatch 421 with water. A thin coat is fine for the first coat, you just need to cover the Inswool/Kaowool. Let it dry overnight then start your forge and heat it to a dull red then let it cool. Now apply a second coat. Two coats is usually sufficient and should last a very long time. Of course, you can apply more coats if you like. It's really pretty forgiving with the curing times and temps.
Iron Dungeon Forge