Re: Sparton Horn Restoration -- My 2 cents worth
Posted by Marco on July 20, 1998 at 01:10:13
In Reply to: Sparton Horn Restoration -- Here We Go Again....
posted by Gordon Biggar on July 18, 1998 at 20:32:02
| Well I just returned from camping for the weekend with the family and have to get back in the middle of everything!So here's my two cents First, the problems associated with blasting are both the media and the machine.Many blasters use a very large sand or garnet media which can induce craters the size of rust pits which can severely increase the need for primer fill and sanding.On sheet metal this generally will cause heat, warpage, and surface hardening of the metal making it brittle.The other extreme is fine glass bead which will rarely cause any of the above side effects to a noticable degree.Glass bead doesn't have the sharpness to the media to get the surface as clean (it will look clean) as sand.A rusted item after being properly blasted clean can be placed in a dry place and show no signs of rust for months (or even years).This is easier to achieve with sand.There is fine garnet (#000 I believe)That will work similar to glass bead but provide a cleaner surface.There are twomachines or methods.MOST cabinets are syphon feed which is slow and tedious but produces less heat and associated side effects.Most stand alone units and a few cabinets used commercially are pressurized which sprays the media more rapidly or a higher concentration which requires more care to avoid the same side effects. Any sheet metal which is formed with a compound curve can typically be blasted without problems. The shallower the curve, the more care/caution required.A Model A horn is nearly indestructable by blasting with the exception of the diaphragm.Even the screen will be unscathed with ultra fine sand or glass bead.When blasting items such as doors or quarters, the blasting should be a repeated dusting from an angle to minimize the heat. The surface being blasted will grow and draw outward.I'm not sure if the cause is a peening effect or the heat generated, but likely a combination of both.In most cases the change is undetectable but care must be taken.A coupe/roadster decklid for example will be fine when properly blasted on the outside.The outer skin is visible on the inside through six large holes in the inner panel.If blasted on the inside without the outer skin protected at the upper three holes where the metal has little curve, the metal will draw toward the blasting and leave three perminent (sometimes severe) low spots in the upper section of the decklid. For reference, I had an original radiator that someone had applied a paint that I could not remove with any strippers.I successully bead blasted it clean in a friends cabinet without distorting the thin brass fins or tinned tubes (don't try this at home!!!) Now on to the paint :)The problem with aerosols is the pigments.Black is usually (if not always) a dark charcoal grey.In my experiemce it has a very limited life span.Back in the mid 1970's I did the horn on my '29 Roadster and it was beautiful (almost black), but within two years it looked like a beautiful 50 year old unrestored horn!Not exactly my intention.The aerosols were better then than now! If you are up to the personal challenge I to would suggest trying a little work with a spray gun.I am also afraid of the brand "X" spray guns.Especially if you won't know if problems are from you or the gun!I use a lower volume Binks that is to slow for a production painter but I can control it well. If you have little or no exposure to the prep and paint process you will need someone to explain the basics and from that point it's pretty simple. I'm sure there's more but my two cents has runneth out for tonight! Have fun pondering, Marco |
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