The Volkswagen crankcase is cast from a magnesium alloy (about 96% magnesium). A critical characteristic of magnesium is that it’s highly reactive; it likes to corrode. Upon manufacture the crankcase is usually treated to a chromate bath but the protection is defeated by time and heat.
Since a properly assembled engine based on VW after-market components can give twenty years or more of reliable service, it’s vital that the magnesium alloy crankcase be given some form of protection. For the home-builder, painting the crankcase has proven to be the most practical solution.
Although any good oil-based enamel will serve to protect the case, all forms of paint act as thermal insulators. To preserve the function of the crankcase as a thermal radiator a thin coat of flat-black paint will provide the best results, since flat black has a higher thermal emissivity than any other color.
If flat black paint is not available you may use gloss black. Mixing a small amount of naphtha or even gasoline (!) with the paint will kill the gloss.
Paints intended for high-temperature applications should be avoided. Often called ‘stove paint,’ barbecue’ paint or ‘exhaust’ paint and advertised as being able to withstand temperatures as high as 1200 degrees, these paints get their high-temperature qualities from clay, metallic salts or ceramic frits, all of which make excellent insulators. The use of such paints will reduce the engine's ability to rid itself of waste heat.
Common oil-based enamels can withstand temperatures up to about 400 degrees Fahrenheit, far higher than your normal crankcase temperature.
When painting the crankcase it’s important to keep the paint where it belongs, which is on the outside of the crankcase. This can be accomplished by careful brush-work. If using spray paint, you should mask a
ny area you don’t want to paint. As a general rule that would include any sealing surface or threaded bore.When building just a single engine the masking is usually done with tape. Threaded bores may also be protected with corks, plugs of various types and even dowels. However, people who normally build more than one engine at a time often make up a set of re-usable masks. Some of these can be quite elaborate but I've found cardboard to work well enough.
In Fig 3 you can see the front of the crankcase with the cam plug and the #1 main bearing area masked off with cardboard salvaged from a cereal box (I think :-). Dowels are used to mask the tapped holes to the lifter oil galleries while the main oil gallery has been sealed with a small cork.
The cardboard is held in place by rubber cement. Applied to the cardboard and allowed to dry, it remains tacky enough to stick to a clean crankcase, peeling away without leaving any residue after the paint has dried.
Whenever possible I try to use an existing part or gasket for the mask. In Figures 4 & 5 you can see the anti-splash baffle being used to mask the dynamo base while an old sump plate takes care of masking the bottom. Fig 4 shows a couple of corks and another dowel. Normally, I use an old distributor body and a fuel-pump block-off plate as masks but they've wandered off so I used masking tape.
The crankcase shown here was painted with Rustoleum Flat-Black in a rattle-can, which makes it rather expensive. An air brush works equally well and a quart of paint will do a dozen engines or more.
At my location it takes the paint about a day to cure well enough for it to go into the oven, where it will be baked at 170 degrees for four hours, after which it's remarkably bullet-proof. If you don't have a shop oven that will accept a crankcase you can line a cardboard box with aluminum foil large enough to fit down over the engine and rig a 100W incandescent lamp to go under it. The rising heat will be trapped by the box, raising the temperature high enough to harden the paint in about 12 hours.
If you live in a warm, sunny climate you can also pop the painted crankcase into a parked car standing in the sun. It generally takes two or three days for the paint to achieve the hardness and scratch-resistance it gets from being oven-baked for four hours. (Even a black car will reach thermal equilibrium at about 145 degrees F, a bit low for optimum paint-curing.) But be warned: As the paint cures a greasy residue will appear on the inside of your car's windows :-)
-R.S.Hoover