Finish Plasters for rocket mass bench

Hi all,

I searched the above term and got nothing.

What's the simplest way to finish and seal my rocket mass bench? Can i just seal with with linseed oil and if so how many coats? What can i paint it with because there are some issues regarding finishing which need to be covered up.

It started out as a "month" project. Its been 5 months now. Not complaining.

There's probably a thousand different ways to finish it. Do you like ceramic tile? Throw some thin-set on it and make yourself something artistic. Try searching on "Roman tile floor" and see if that gives you any ideas.

If you like the rough adobe look, you can use a colored stucco as a finish. Then if you get tired of the color of the stucco, you can paint it. Painted stucco is pretty inert, friable bits don't flake off and it's easy to wipe up.

You could use drywall plaster and then bead-blast it with beeswax. I've seen that used as a wall finish in New Mexico and it comes out really nice.

Then again, you could try a pool plaster. That would make sense in a sauna type application, or where you expected it to get wet a lot.

I could think out loud a lot more, but maybe I should stop and ask you what you want it to look like in the end.

Hi John

I appreciate the long list of options..im sure there are even more..

What im looking for is a simple way to simply make my rocket mass bench waterproof and smooth. I would also like to cover up if possible the unevenness of the surface being that i didnt smooth it out as it dried, can a paint finish hide that?

What about simply putting ona few coats of linseed oil, would that do the trick mostly or would i be left now with a shiny sealed uneven surface?

Linseed oil on what? Is it cob?

If it is a soft material (not too much lime in the cob to harden it up), then I would suggest sanding the bumps down. Get yourself a hand-sized chunk of pumice, put a nice flat on it with a tile saw or an abrasive wheel, and then use that to take down the high spots. Then wet it with some water from a spray bottle and see if that brings out any more high spots. Try and get it the way you want it before you put on any sealer or finish coat.

Then yes, you could try linseed oil as a finish coat. As the linseed oil ages on the surface, chemical reactions occur that will polymerize it and that in turn hardens it up and makes it somewhat waterproof. There are two ways to speed up the polymerization reactions: (1) the UV method -- lots of direct sunlight or a black light and (2) the chemical method -- some peroxide that will attack the unsaturated parts of the linseed oil molecule and give it a kick to polymerize. Benzoyl peroxide should give it that kick, and it's readily available at your local pharmacy -- as zit medicine.

Hi John,

I failed to mention, the bench is all cob and its hard now but i forgot i can wet it..ill try that, sand it down when its damp, then wait for it to dry before applying linseed oil.

What can i paint it with in an earth tone? I dont want white..

Karl : Horse Shit ! - Sorry, I could not resist, if you really want that natural look, and want to use old time materials and be 'organic', that is what you should use,
the linseed oil should darken it a little darker than Eggshell ! or check with a local potter, when you explain what you want it for and show them a few pictures on
your phone, you will have probably made a convert ! Big AL

There's all the colors in the world down the tile aisle at your local home center. Try using an unsanded tile grout as a finish plaster. If you like a little more texture to it, use the sanded kind. It may take a little trial and error to get exactly what you are looking for, but this is not the Middle Ages where you have to settle for horseshit brown as your only choice.

Once you get what you want, you can keep it the same color if you seal it with tile grout sealer, or you can darken it a couple shades if you seal it with linseed oil.

Horse shit...interesting...so far ive only used it for compost. Think ill pass, but the linseed oil sounds like the possible winner here. Ill look into tile grout but first i have to figure out how to say it in Spanish..

"Yeso por los azulejos"

Aha!! Gracias!!

Does anyone know if it would be possible to paint it with flour paint?

Hey Tiffaney,
I’m struggling to understand what exactly you’re asking, I’d like to help.
What is it that you want to paint? A cob bench?
What exactly is flour paint? And what are you trying to achieve?

Hi Benjamin and thanks for replying and asking. Flour paint is a natural, traditional Swedish painting method. It's mixed flour, water, pigmentation and linseed oil. I really like the feel it, so I thought it would be nice for the final coating of the rocket mass heater bench.

On this thread, people have said linseed oil. And I can't imagine there being a problem with pigmentation. But the floor? Will it eventually burn and will our bench be like burnt bread? Or will it be okay?

Hey, thanks for clarifying. The flour is essentially for structure in the mix I imagine. A lot of lime paints use fine sand or other powders for that. In the combination with linseed oil I can’t imagine a lot of problems. But I haven’t used it, so I can’t vouch for it. You could try a patch and worst case sand it off if it discolors in an unwanted way.
Please share your findings! And good luck!

Hi Benjamin,

The flour in flour paint isn't like fine sand,  the gluten in the flour is the glue that holds the pigmentation.

Once we're using the stove this winter, I'll paint a small spot of the bench to see what happens. Thanks for the suggestion. And I'll come back here with the results

Good day to everyone