Linseed and paint

Daniel Mazur - Saturday, July 31, 2010 - 7:03am

If I put a 50/50 mix of mineral spirits/linseed oil on an iron piece I'm working on.

Is it possible to paint it?

No powder coat due to the cost involved.

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Dan, The linseed/mineral

Rich Waugh - Saturday, July 31, 2010 - 7:37am

Dan,

The linseed/mineral spirits mix is just a "poor man's varnish" so yes, you can paint over it. You'll need to wait until the linseed oil has fully polymerized before over-coating it, though. Depending on the particular batch of oil you used that could take anywhere form several days to a couple of months. If the linseed surface feels hard and doesn't easily dent with a fingernail then it is hard enough to overcoat. If it isn't fully hard before you overcoat it, it will never harden properly and the paint over it will be very easily damaged.

Don't use a lacquer-based overcoat - it will lift the linseed oil finish underneath it. Stick with regular oil-based paints, alkyd enamels and the like for the overcoats.

When you're planning to paint a piece, particularly for exterior use, I recommend that the first coat be 95% zinc cold-galvanizing primer. Not just some "zinc rich" paint, but the real stuff that is 95% zinc powder in a bit of vehicle to make it stick. That stuff is almost as good as hot-dipped galvanizing if you apply it over clean bare steel. After that dries I usually give the piece a coat of neutral oxide primer like "red oxide primer" and let that dry thoroughly. Then two coats of your favorite color and the piece should be good for several years outdoors with no maintenance.

The reason for the neutral red oxide primer after the zinc is twofold: most paints adhere better to it than to the zinc, and the reddish color over the gray zinc paint makes it easy to spot any areas you missed.

Lots of blacksmiths use a homemade clear coating of boiled linseed oil, beeswax, turpentine and a bit of Japan dryer (cobalt lineolate) to speed drying. My personal preference is to simply use a high-quality satin spar varnish with about 5% Penetrol added for durability. Sometimes, when I remember, I add a bit of alizarin crimson artists oil pigment to the varnish as it gives the finished steel a barely-noticeable warmth to the appearance. The reason I use the store-bought varnish instead of making my own is that I don't have to go to the trouble to formulate it and it outlasts the homemade stuff by a wide margin. What can I say - I'm lazy. (grin)

I heartily agree with you on not using powder coating. I recommend against using powder coating for any exterior steel work since there is no way powder coat can be "touched up" when (not if) it begins to fail. Industry likes powder coating because it is more profitable for them in mass production but properly applied, a good paint will outlast powder coating by a long ways. They don't powder coat cars, do they?

That's a pretty long-winded answer to your question and probably more than you wanted to know, huh? I just can't help myself, I'm verbose. :-)

Rich

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Big Thanks

Daniel Mazur - Saturday, July 31, 2010 - 7:53am

Thanks Rich.

That's not long-winded at all.The more info I get the better I'll be at this. One of the things that came up was could I do something to leave it unpainted for outdoor use. I didn't think there was, so I left that option out. But what I'm getting out of this is if I follow one of those methods you mentioned, then I can leave it unpainted?

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Wellllll...it depends. You

Rich Waugh - Saturday, July 31, 2010 - 11:13am

Wellllll...it depends. You could just use the varnish outdoors, provided that the client would be diligent about maintaining it. That means re-coating it every couple of years if it is in a relatively protected location. If the piece is to be located in full exposure to all the elements then the varnish won't hold up for more than a year, probably.

Even the best varnishes/clear coats are susceptible to degrading by UV, rain, ozone, etc. If you use automotive clear coat with a urethane hardener additive and the customer keeps the piece waxed with good car wax, it might last two or three years - or not. Clear finishes are weak by nature, there's just no getting around it.

What I usually do for a client who wants the "as forged" look of bare steel is this: I use the cold galvanizing primer, the red oxide primer and then topcoat with light gray automotive paint. After that has dried/hardened I then put a coat of clear over that and let that dry. Then I spray it with deep charcoal gray and rub some of that back off the high spots with steel wool or Scotchbrite to expose the lighter gray underneath. (The clear coat between the two color coats gives me a "margin of error" in case I get a bit aggressive with the rubbing off.)

Once I have the piece looking like bare steel I give it a couple of coats of satin clear. The end result is something that 99% of the people cannot distinguish form unpainted steel but that has a solid coating system that will prevent rusting for several years and can be maintained with just a bit of car wax from time to time.

Properly finishing a piece of forged work is almost as much work (sometimes more) than making the piece. I build that into the price. If I send a piece out with no finish I have learned by bitter experience that in a few months the client will be calling me up whining that it is rusting, even though they were warned this would happen. From that point, things only go downhill. I'd rather avoid that.

Rich

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