Silicone and welds

I am working on a gasifier and plan to make a seperate thread when i have more pictures and progress.

Anyway I have reached the point where i have one more weld to make then its time to fill the unit with water to test for leaks.

How well does Hi temp silicone 600F rated work for patching pin hole leaks? I see people mention going over there welds with it quite often but to me it seems like my gasifer will be hotter then 600F where most of my welds are.
I dont want to use it and have leak after 5 minutes and waste all that time. Plus risk a exsplosion.

I also picked up some woodstove sealant that is rated 2000F and is like a hightemp motar not sure if this stuff will adhere long term but worth a shot.

The hearth of my unit is a thick rusty old rim of some sort. Makes for difficult welding to thinner 55 gallon drum material, atleast when everything is air tight.

Basically i am asking when is silicone worth using and had anyone used the stove cement stuff sealant? Do i just need to get out the grinder and make everything perfect? ( which could take a while depending on how many unseen pinhole leaks i have)

Thanks to all who chime in ,love how helpful people are on this site!


Take a pic and highlight where you’re most concerned about leaks. If it’s in the bottom barrel like the exterior, you can patch it with high temp RTV, and then a layer of aluminum tape over. Same works for pipes. The rtv eventually will come off those sorts of areas. If it’s around the hearth or air jacket it will probably be hotter than 600F.

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Hello Taylor .

If you have a few just tiny pin holes I wouldn’t worry much about them .

I often use the silicone on the gasifier and it all depends on what part of the gasifier as how well it will hold up .

I have never used any furnace cement .


I didn’t even see the furnace cement part.

I’ve tried to use it, I’m not sure if it’s my luck or what, every time I try to use it in my caulk gun it comes out grainy and too stiff. Hard to spread and it doesn’t adhere very well. I wanted to use it as a gasket seal for a lid but it just kept falling apart. It’s like Play-Doh.

I’ve attempted with 3 different tubes of the stuff.

When you’re welding thick to thin, focus your arc on the thicker part and just whip it over to the thin for the weld pool to stitch it in. I do a sort of tempo of twice as long on the thick as I do the thin. 1, 2, whip to the thin then diagonally go back to the thick to bring the weld pool back over. I find it easier to do thick to thin than thin to thin, but I work with some really poor quality junk.


Someone had to ask for pictures​:expressionless::joy: Kind of embarrsed of the crooked ash catch alinement didnt cut my hole straight on the lid/connector and couldnt find any piece that would cleanly fit.




You have a lot of start stop welds Taylor. I would use a stiff wire brush on an angle grinder and clean them really well. Knock off any boogers with a grinder and run a cover pass over what you have. You should be good or at least be able to see any problems I use the furnace cement on the clean out doors on my wood heater. It gets brittle and can chip off. I’d vote no on that. It’s hard to say from the pictures just how your reactor section is made internally. If you have some kind of shelf to hold ash right about where I assume your hearth would be then the ash will insulate that weld area enough that a skim coat of RTV would be fine. No reason to be embarrassed. I wouldn’t kick it out of bed.


Maybe i am too much of a perfectionist but i won’t use silicone on a new build. i see silicone as a way to get me home to a welder or a way to increase the life of an old part. I would grind down the leaks and weld them till i got them all. Also i have had two engines ruined by silicone so that might make the a little biased against it and for a good solid weld.


Hi Tom Thanks for the advice. Yes i did start stop alot. I should do more grinding before i tac up.

And yes there is a crude grate under the hearth suspended by chains that is both adjustable and removable.

On a side i did a light test in close to complete darkness by dropping a bright latntern stile flashlight in the hopper. I found 2 leaks that where easy fixes. If that is all im happy. Was anticipating alot more.


I am not sure if it is more heat resistant than silicone, but one way to seal pinholes in things like tanks and resivoirs is to heat the pinhole area and put a drop of Loctite on it.

I’ve offered this up before:
Just over braze your steel weld.
You know, low heat, brass filler rod heated then dipped into flux powder. Work your puddle with the torch tip. Dip, dip, dip the filler rod to move it along.
It’s quick. It’s easy. It is corrosion proof to woodgas corrosions.

As TomH said the really HOT internal seams will be ash and tars self-sealing.
The not-so-hot outer jacket/filter train seams; and other seams do not get hot enough to melt the brass.

Only Mike LaRosa and me; it seems ever used brazing on wood gasifiers.
Steve unruh


Brazing can be done on the road as well, with MAP yellow can torch and some fluxed rods bought at any tractor supply store.


Dam, I hate this subject. I built an outside boiler. I spent about 3 months of spare time, pressurizing the water tank portion— then crawling into the fire box and using soapy water, checking for little bubbles, marking the spots, dropping the pressure, and going back into the fire box to weld the marked leaks. Then air the water tank back up and go again.
First suggestion, test with a light in the dark like you already described. Light inside, your head outside or vise versa.
Then the water test. Grind off where the leaks are. Before you go on, inspect the grownd area for any black specks (slag). i never had any luck welding over slag specks. If you see a speck, using an awl or nail sharpened to a point, and a small hammer, chip/pick all the slag out.
Now you can cover the weld over with another weld— or I always thought a braze would flow over and fill better BUT I have never had any good luck with that.
Silicon is only for quick and dirty repairs. Good luck–that helps most. TomC


Here is what I have learned from the experience builders that build long lasting charcoal/wood gasifers. If air can enter into the firetube other then the nozzles this could start a failure of the firetube. After the bottom of the firetube the hot gases will ignite if air is added causing a heater effect in the gasifer. Then it cools down to a point that the air will just mix making a explosive gases.
To avoided this weld and make sure you do not have leaks where it needs to be air tight in the high heat areas. Only use silicone in low heat areas, but it is better to weld the parts.


You can’t get the metal you are welding too clean. People cause themselves more misery by welding dirty, rusty, metal than they know. TomC is right about getting any slag inclusions out as well. Scratch awl, little drill. Get them all out. Needle Scaler is a worthwhile investment. Even a cheap Harbor Freight one.


Tom, took me a decade to fugure this out. A decade of cursing and chickhen poo wealds. You are absolutely right.


Jacob can i ask what happened when the engines were ruined from silicone?


I acted like sand paper on the rings best i can tell. Sometime this week i will finish pulling one apart I will post some pics.


I had to learn this for a class I took in the Army. Plugging holes with a caulking chisel. Now I save all these weird chisels I find around here, because I know this was a steam powered industrial site and these were actually important tools in their day.


I have always wondered how they sealed the seams on those riveted boilers Bruce. We use to have tools that looked alot like that only somewhat bigger. We called the hell dogs. Used to bust the heads off rivets. Rivets were obsolete years before I started working Iron. Still a lot of guys that had worked in the rivet gang. A lot of interesting stories.