I built my first gas forge for knife making almost 10 years ago and it's still going. I have been doing more smithing projects and the small forge was really limiting me. Been having to use my O/A rosebud way too much.
When I built my knife forge, I used 2 burners and soon found out that was overkill. I removed one and plugged the hole, so I had a burner. I had an old 11gal. air tank with a pin hole leak, so I had a body. Scrap metal shelving to make the ends.
I have helped 3 other guys build gas forges and kept all the leftover material. The 1" Inswool was 6" to 8" wide and about 24" long pieces. I had about 6# of Satanite and small amounts of ATC100 and Plistix 900F.
I cut the ends of the air tank and ended up with a 12"x14" body. I used my hydraulic jack to make an oval shaped cylinder. Cut some of the shelving to make the ends and a tray for soft firebrick sliding front door.
I ended up with an interior oval approx. 9"x5"x11 1/2". About 460cu/in volume. To give my venturi burner a little boost, i used a small squirrel cage fan I found in a dumpster years ago and some 2" vacuum cleaner hose to provide extra air.
All comments and questions are welcomed.

awesome use for scraps man.

Did you use both the Plistix and ITC 100? Let me know how you used the products and did you notice anything to comment on? After you respond I will post further.

Wayne, I didn't have enough of either product on hand for a final coat, so I mixed what I had. I would guess 60/40 mix ITC 100/ Plistix. The only thing I noticed was the mix seemed easier to apply than either product straight. I used a foam brush to apply the mix. It worked better than a bristle brush. I will let you know how it holds up.
This project was somewhat of an experiment to see what I could build with the leftover material I had. I knew the single sidearm burner would be a push for the volume. Adding a bit of air let me get welding heat at 2psi propane with a neutral flame. I can hold forging heat at 1psi with the air reduced.
One thing I have learned is to add water with a spray bottle when mixing the coatings. (satanite, ITC 100, Plistix) Start with dry powder, squirt, mix, squirt , mix- until you achieve the consistency you want. It is a lot easier to me.
Mike

so i am gonna be casting with a refractory cement for the top and bottom of my forge then the blanket around the walls. I should then coat this with a coating product like plistix? if so where could i get it from,its alittle over 2 square feet on the inside of my forge so however much would be necessary.

I would put the blanket then over cast with insolating castable (Kast-0-Lite) covering all blanket, then Paint on Plistix. I can supply all of these products. Just e-mail me or call.