GHT UPT
Construction Log:
Bring out the Finish


Created: May 2007

 

It's said that cast iron is wonderful yet messy stuff. Once machined, it is ready for use—unless you decide that form is just as important as function, in which case it needs some "finish" applied. The quickest approach is a swift spray of paint from some pressure pack can of an appropriate color (traditionally green for workshop equipment). But castings can be a bit rough externally. Defects range from little pimples and pits to sodding great discontinuities caused by careless alignment of cope and drag during casting. Suppliers of castings will often take to the part with a grinding wheel to "correct" such misalignments. I rather wish they wouldn't. It just makes the job of achieving that award winning finish harder.

In his book on the QUORN, Prof Chaddock refers to this process as "bringing out the finish". This is not, as it sounds, a black ages practice akin to bringing out your dead. What he means is lowering the high spots and raising the low spots until a surface is arrived at that will take a coat or three of paint and look smooth, uniform, glossy, and as good or better than any commercial product.

The basic process is described in the finishing section of the Quorn Construction Story. Although that was written over ten years ago, it has endured so well that I saw no point in changing it apart from selecting a cheaper metallic epoxy filler product. Rather than repeat those instructions, follow the link to review the Quorn finishing section and read them in conjunction with the photos below.


Photo 78

Photo 79

Photo 80

Photo 81

Photo 82

Photo 83

Photo 84 (Before)
 
Photo 85 (After)

Having forgotten to take a "before" shot, ignore the thinggy in the base of Photo 84 and take note of the following:

All these minor sins were corrected in the finishing process. Now a few parting observations:

  1. Be sure to clean the item thoroughly before the first application of epoxy, and after each filing/sanding stage. An automotive agent called "Prepsol" is good for this.
  2. Modern metal filled epoxies are probably stable and will not shrink over time, but cellulose products applied over them may, causing the painted finish to crack (like my mega-expensive Myford, dammit!) therefore, use as little as possible. It's ok to trowel it on thickly as long as you file/sand most of it off. Until you get to the stage of applying the etch primer base (Photo 83), you should always file/sand back until bare metal is showing in the high spots (Photo 82).
  3. The epoxy will clog your files. Use an old file, or one that is about to become instantly old . Periodically use the corner of a scrap of brass shim to dig the lumps out of the file teeth and follow up with a jolly good "carding".
  4. Don't use your fingers as a sanding block! They are too soft and will not take off the high spots. Small blocks of wood should be used, contoured if necessary to get into concavities. This is probably the most important and most frequently ignored secret of achieving a winning finish.

 

 

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