Finishing Castings and Final Painting

[BronxFigs]

After all the machining, tapping holes, etc. the builder has to then paint the castings. Often the castings have a rough surface. I would like to know how these rough, casting surfaces are prepared for final painting, and what kind of paint should be used to give that glossy, smooth surface?

I'm guessing that the castings are either filed, sanded, and then filled with some kind of glazing compound, then sanded again, until perfect...then, primed, and sprayed with paint. Are these paints, the kind that need to be baked; self leveling, high-gloss enamels?

Just wondering.

Frank

 

[rhitee93]

Hi Frank,

I struggled with this last summer, and then documented how I finished some castings in my PMR build. The discussion on castings starts in post #134. Hopefully this will link you right to it:

http://www.homemodelenginemachinist.com/f31/pm-research-no-1-build-17259/index14.html#post186990

 

[OrangeAlpine]

Having stripped the finish off of several of old castings, it seems they went from rough casting to filler, then primer and paint. I have no idea what filler was used "back in the day". But it was pretty good stuff and they were very generous in its use.

I suggest that for the filler you NOT use Bondo. It is terrible crap. Prone to air voids, uneven hardening, hard to sand and is bad (or good, depending on your viewpoint) at filling sandpaper. Go to an automotive paint supply and get something that is good. You will be amazed. I have used Evercoat and it is marvelous stuff. There are probably others that are just as good, but it isn't Bondo.

Bill

 

[GWRdriver]

I once read and explanation of how an old fellow (British) got such a beautiful finish on the cars here restored. "I apply thirteen coats of paint (or lacquer) . . . and rub the first twelve off."

I use an automotive body filler (Ultra-Fill, Sherwin-Willams) which is actually a very heavy-bodied sandable primer (NOT a Bondo) and I often see the need to thing this down with solvent. Once this is applied and sanded smooth I give it one thin coat of conventional automotive primer, then I start applying paint. Three coats (first two rubbed) usually gives me enough finish for what I'm doing. The problem I've encountered is finding high quality heavy pigmented enamels in small quantities. I recently bought a small tin of a US-made sign painter's enamel, brand name "One-Shot," but I haven't yet used it so I can't file a report.

None of the above requires "baking," and baking would probably do more harm than good to the filler from rapid shrinkage and wrinkling. Beyond that, "baking" doesn't enhance the basic performance of the paint, the only thing it does is accelerate the driving off of the vehicle (the volatiles content) and this can be done at 150F or less. Depending upon the paint, higher temps can damage the paint by changing the color, wrinkling, burning, etc. IMHO this is a place where more is not better.

 

[BronxFigs]

Thank you for the quick and informative replies and comments. Mostly prep work, sanding, filling, and sweating. Now, I know and have seen, just what it takes to get that beautiful finish.

Frank

 

[deverett]

I know that GHT used to advocate painting castings before machining. The idea was that there was a good cut line from the painted body to the machined part. That approach may work on some shapes of casting - like flywheels, but there are a lot where the paint would get damaged with the holding while machining. If you are good at masking, then forget the above!

The use of an etching primer before painting is a good idea to ensure keying of subsequent paint coats. You also want to paint - or at least seal with a couple of coats of paint before operating your model, especially where there is a likelihood of oil getting onto the bare casting. It's a lot easier to wipe oil off a sealed surface than try to get it out of a porous casting.

Dave
The Emerald Isle

 

[goldstar31]

Old fashioned it may be but GHT's mate gives a blow by blow write up of how he did his Quorn. Prof Dennis Chaddock is your man.