Baking enamel

[daniel_leavitt2000]

OK, I'll admit this isn't actually train related.

While laid up with a recent surgery, I took up a new hobby: watchmaking. This requires only a small supply of tools (most of which I had) and a small workspace, so I could work in bed.

I won't bore you with details, but one of the things I wanted to do was change the watch hands from metallic silver to blue. Normally this is done by heating the parts and cooling them. But I don't have experience with that so I decided to paint them. Fist coat was Floquil Bright Silver followed by Alclad II Candy Electric Blue.

My first attempt looked good. I dried them with a hair dryer with heat set on high. In the past, this has worked well on brass engines as the large shells absorbed the heat. But watch hands are very small and the lacquer didn't "bake" on as I am used to and the parts scratched when put back on the dial.

Right now I have the newly repainted parts in  a toaster oven set to 200. But I don't know what the sweet spot for enamel baking is. 200? 260? For how long?

[nkalanaga]

I Googled "baking enamel paints", and couldn't find any consensus on the temperature.  However, all agreed that it's critical to wait for the paint to dry until it can't be smelled.  On a watch hand, I'm not sure there's enough paint to BE smelled, so that might not help.  The only paint I've baked is Scalecoat, and I let it dry a week before baking it, as it seems to take that long before it's not sticky to the touch.

Temperatures suggested ranged from 175 F to 225 F.  I usually use 200 F, in our kitchen oven, which probably isn't that accurate.  It seems to work on etched brass passenger car sides, which is all I've baked.

Times recommended in the sites I found ranged from 20 minutes to an hour, with the least useful being "until it's done".  At 225 F or less you won't harm brass or stainless steel, and the paint won't burn, so leaving it in longer won't hurt. 

For brass locomotives, several commenters warned to look for soft metal weights, as they can melt at these temperatures, and literally run out of the model.  That wouldn't help the oven either!

[peteski]

Floquil might be an enamel but I'm pretty sure Alclad II is a lacquer.
Some random thoughts:
If the hands were already silver why didn't you just spray the Alclad II directly over the metal?
It might be that the Floquil was not fully dried/hardened under the Alclad during your baking process.
Also, I doubt that you will be able to make Alclad II hard enough to resist scratching.
As far as the heat is concerned, small parts get just as hot as large parts when placed in a stream of hot air.  They heat up quickly and also get cooled off very quickly once removed from under the hot air stream.

[Iain]

Oddly enough, baking the parts is precisely what you would do to change the color.  Steel will turn blue at the right temperature, depending on the alloy used.

[peteski]

Oddly enough, baking the parts is precisely what you would do to change the color.  Steel will turn blue at the right temperature, depending on the alloy used.

I suspect there is more to it than just heating. I think it needs to be coated with some chemical which will cause the color change at a high temperature.  But I'm no expert either . . .

[Iain]

No, no coating needed.  Try it, take a piece of steel, polish it, and hold it (close) over an open flame.

[wcfn100]

Try it, take a piece of steel, polish it, and hold it (close) over an open flame.

That's not baking.  Baking is indirect heat.

Jason

[Missaberoad]

/>
This is what John is talking about, and what Daniel is trying to emulate...
skip to 8:10 to see the bluing process in action...

also probably more then a few here will find Clickspring's channel interesting...

https://www.youtube.com/channel/UCworsKCR-Sx6R6-BnIjS2MA

[muktown128]

Short answer:
I wouldn't bother baking these paints at temperatures above 200 F. 
Higher temperatures won't help much (diminishing returns).
If the paint seems soft, then bake longer (30 minutes -> 1 hours; 1 hour -> 2 hours). 
You'll probably need to experiment a little.

Long Answer:
Lacquers dry through solvent evaporation. 

Air dry Enamels dry through solvent evaporation and cure oxidatively (post-cure).  This is typically a slow process and can take a few weeks to fully cure for industrial enamels.  C=C bonds become C-C bonds as resin molecules connect with each other or cross-link (alkyds resins).  Typically metal driers are used (Cobalt, Manganese, Zirconium compounds) to speed up cure.  Cure can be temperature dependent.  150-200 F being the typical range for optimum results.  Longer times at lower temperatures can accelerate post-cure.

True baking enamels are formulated with different resins that react (chemically cross-link) when exposed to heat.  The cross-linking resin is typically melamine.  Bake temperatures are typically 200-300 F.  The optimum bake temperature will depend on resin selection and catalyst level. 

Allowing the solvent to evaporate before baking is important for a couple of reasons:
1.  Solvent trapped in a coating can cause bubbles or blisters.
2.  Solvent vapors are flammable.  Home ovens or toaster ovens are not designed to vent solvent vapors.  Enough solvent vapors exposed to an ignition source can explode or cause a fire.  This has happened a few times where I work, even when using ovens designed for baking paints.

Metal mass does contribute to how quickly a part heats up or cools down.  I have been to several manufacturing plants that make construction equipment, agricultural equipment, automotive wheels, etc... and metal mass affects rate of heat up and cool down.  I have seen temperature charts that record temperature versus time on parts of different thicknesses, so this can be measured.  You probably need a pretty high volume of air blowing on a part to overcome this.  Different part thicknesses may not matter much with small models, but when painting parts that weigh a few pounds versus hundreds or thousands of pounds or parts 1/8" thick vs. 3" thick, this is a big deal.

Regards,
Scott