12.14. Brush Painting Pointers

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Brush Painting Pointers

Gleaned from “The List”

Since I worked for a time in the coatings industry I can speak to this. Alkyd ("oil-base") paints and varnishes are made from 3 basic types of resin, characterized by cure time: fast alkyd, medium alkyd and slow alkyd. The cure time is the amount of time the resin takes to polymerize molecularly as opposed to the dry time which is the time the paint takes to throw off its solvents (basically this is evaporation time). Generally speaking the longer the cure time the more durable the coating is. Since in industrial production work a quick cure is desirable (and more costly) it follows that the slow alkyds are cheaper. Since Rustoleum is made primarily for the "consumer" market it is naturally made from the cheapest ingredients possible to maximize profit. This is why it has an annoyingly long cure time but this long cure actually leads to a more durable finish than one would get with a quick dry, quick cure industrial enamel. Just set your painted parts aside for at least 2 or 3 weeks to cure fully and you'll have a pretty darned tough coating. Rob is correct about a thin film being more durable than a thick film, generally speaking, and the cure time will probably be less as well. A thin film is less prone to chipping. High gloss finishes are generally more durable than flat or satin finishes. Metallics generally don't hold up as well as solid colors unless given a clear varnish coat.

When painting surfaces, especially metals, thorough preparation is the key to good results. Before applying any coating clean the surface thoroughly and afterwards rinse with lacquer thinner or MEK (methyl ethyl ketone). Make sure the area is very well ventilated when using any solvent or paint and keep the stuff off of your skin. These solvents are all highly toxic. Buy a big box of latex medical examination gloves, they're cheap. I use them any time I'm working on my bike. Use the same brand of primer and finish coat and follow the manufacturer's instructions to the letter. Make sure the paint is thoroughly mixed. Pour a little off the top of the can into a clean container so there will be enough room in the can to stir all of the pigments up from the bottom of the can without making a mess. Then pour the paint back and forth from one container to the other to get a thorough mix. This is what paintmakers and painters call "boxing" the paint.

Don't buy cheap brushes. Go to a paint store frequented by professional painters and buy professional grade brushes. These will be easier to use and will leave fewer brush marks. They are expensive but if properly cared for they will last for years. I wouldn't try to speed cure time by pointing a heat lamp directly at the painted surface (unless baking is recommended by the manufacturer) but painting should be done at a temperature above 50 degrees Fahrenheit as a minumum (or whatever the manufacturer recommends) and a warm environment during cure will definitely speed things up since molecular reactions are sped up by heat.

Happy trails, Mike

If you want to paint your frame with a brush, then use the slowest drying enamel you can find. Ordinary enamel (melamine formaldehyde) does not retain it's gloss after prolonged exposure to the sun. Having said that, I live in the Land of Oz (skin cancer capital of the world). Also, buy a brush that is marked "pure bristle". "Bristle" is name given to the hair off a particular breed of pig that is native to China and is the only type of hair you would use on anything important. Brush on the primer and sand it down the same as you would have if it'd been sprayed on. 360 or 400 wet (the black paper) is the stuff for this. Spray a guide coat* of the opposite colour (black on white, etc) prior to sanding. If done correctly (and only if you use a solid colour) no one should ever know you brushed it.

Dave R

  • A guide coat is when you spray a very misty coat of paint over primer in a colour opposite to the colour of the primer -white over black, blackover white, grey over red, etc. The idea is that as you rub the primer, the guide coat disappears. It will be rubbed off the high areas first and the low areas last. When it all gone, the primer is perfectly smooth and ready for colour.

Dave R.


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