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Painting & refinishing iron castings

Joe Michaels

Apr 3, 2004
Shandaken, NY, USA
I am in the process of rebuilding a small stationary steam engine from 1880's- 1920's. This is a guess on my part based on design features of this engine. The mainframe of this engine is a cast iron 'box bed'. The cast iron was slathered with many coats of paint, top coat being black in color. The cylinder never had a good solid mounting to the mainframe as there was no way to get any fasteners to tie down the mounting flange one side of the cylinder. At best, this side had 1/4-20 screw shanks cutoff with a bolt cutter for 'locator dowels". The overall cylinder mounting was sloppy at best. There was no way to get a studbolt and nut on this side of the cylinder mounting, and there was a jagged cutout in that side of the frame for the exhaust steam elbow. I opted to reinforce the frame from the inside with a steel doubler plate, secured with screws and brazing. I then brazed a fitted lug to the mainframe under one of the 'dowel' holes & milled a pocket for nut/wrench clearance. Brazing let me create fillets and 'wash' the bronze braze alloy to look like the lug was part of the original casting. Some work with a die grinder and a run with the air needle scaler created what looks like part of the casting.

The mainframe never had any mounting feet in its original casting. At some point in the murky past, steel mounting feet had been rivetted to the bottom of the mainframe. These were insufficient for a real working engine and situated to interfere with a properly sized flywheel. I made proper mounting feet from steel flatbar and fastened them to the underside of the mainframe with countersunk screws, as well as welding to the remaining portions of the original riveted steel mounting feet. I created fillets between the mainframe and the new mounting feet using bronze brazing.

For smaller detailed work and cast iron repair or modification, oxyacetylene brazing is my preferred process. It creates strong joints, and with some manipulation of the torch flame, can be 'washed' to create smooth fillets. I could not see using automotive body filler (aka "Bondo" or similar) on this casting.

With the brazing, all the slathered-on paint in that area got burnt off. I used the air needle scaler plus a die grinder with wire-wheel to strip the rest of the paint. This disclosed what looked like the old 'red lead' primer. It also disclosed the casting, while having some ornamental detail, was rough and never really 'fettled' in a number of locations. I ran a die grinder with 60 grit abrasive discs over the casting to knock down the roughest areas. I then re-ran the needle scaler to get a uniform surface. This left the matter of priming and painting. I had a spray can of a "Glyptal' type electrical insulating varnish (Dolph's 631, possibly). It is an orange-red high-build coating used mainly for electrical insulation work. I had used this spray for repairs to insulation in my old Lincoln engine driven welder generators. I recalled Glyptal was often used to coat the insides of cast iron gear cases and oil; reservoirs. I sprayed the insulating varnish on the engine bed casting. The 'substrate', being a clean casting, with a 'frosted' finish from the needle scaler and the sand cast surfaces, enabled the insulating varnish to form a very good adhesion.

The insulating varnish acted as a high-build primer and really smoothed out and blended the casting surfaces and my brazed modifications. I let this cure for a couple of days and decided to try spraying a top coat of Rustoleum Gloss Black from a rattle-can. I figured if the paints were incompatible, the needle scaler would make short work of stripping it off. One coat of gloss black spray went on. The finish is way better than I expected, a nice glossy and smooth enamelled appearance with a few hints that there is a rough casting under it. The brazed lug is on the exposed area of the casting. With the finish on it, there is no tellin g that it was not a part of the original casting. I left the orange-red "Glyptal" finish with no topcoat on the inside of the engine box-bed. It is what was done on full size engines and powerplant equipment. It is also what was done inside some geared-head machine tools. Glyptal can be purchased in aerosol spray cans or in cans for applying with a brush. It is resistant to oils and some fuels, and is one tough coating. It did a better job than the automotive 'sandable primers' on a rough iron casting. The Dolph's 631 spray is about the same color as the original red-lead primer, and accomplishes what the red-lead did in filling and smoothing the cast surfaces.

The use of Glyptal (or equivalent) and bronze brazing are consistent with what the oldtimers would have done on this type of work. I may be off base in saying this, but knowing the approximate age of the engine, I feel better about using somewhat old-time work to rebuild it.