The most important tool a painter has is his palette knife. It serves at least four purposes.
1. You use it to mix pigment and medium to make paint when a glass muller is not available.
2. You use it to mix paints together either with more or less medium, more or less thinner, and other paints to create custom colours.
3. You can use it to apply paint directly to the canvas. There are a lot of reasons to do that which include applying a thick layer of paint, the great advantage of speed, sparing your brushes from heavy duty work, and special techniques such as painting with the knife itself.
4. The fourth reason you need a good palette knife is that you may need to scrape off a layer of paint that has been too thickly applied or just doesn’t look right so that it can be painted over or simply as a way to remove unwanted texture from the painting.
One reason that palette knives are great it that they are easy to clean, unlike brushes. Just rub your knife with a cloth until all the paint is gone. It doesn’t matter if the paint is dry or not as it will still rub off.
There are endless varieties of shapes of palette knives available. You can get a variety of them for your collection if you paint with the knife often, but I find that having one is enough most of the time.
I don’t like plastic palette knives because they break and usually have rough edges. You can improve the edges with some fine sandpaper, but you might as well get at least one good metal knife even if you like to use plastic ones. Plastic knives can be useful because of the different shapes you can get cheaply so if you like to paint directly with knives a cheap set of plastic knives can be handy.
A good palette knife must be flexible enough that it won’t break while mixing and so that it won’t cut into the painting unless you want it to. The knife should be tempered so that it responds like a spring and so that it doesn’t develop a permanent bend. It also needs to be thick enough that it can be used for heavy duty mixing. For these reasons good metal palette knives are made from regular carbon steel which tempers more properly than stainless steel.
While carbon steel can be tempered to be more flexible than stainless it does rust more easily. If you are using your knife to mix water into your paints make sure you clean it and then dry it well before putting it away. The oil from oil paints will help protect the knife from rust, but check your knife occasionally to see that no rust is developing.
If you do see a spot of rust immediately rub that spot with your fingernail and the rust will come off. If the spot has set in you may need to use a scrub pad or a piece of super fine sandpaper. After the rust is gone apply some oil to the knife from your paint kit to prevent any remaining rust from doing more damage.
Do not oil your knife with with synthetic oil like gunsmiths use because that kind of oil in your paint will ruin it, and besides, that stuff stinks. Oils that do not dry entirely like mineral oil and olive oil, while great for kitchen knives, fighting knives, and swords, will also ruin your paint by preventing the paint from drying. When oiling your knife just use a small drop of some painting oil like linseed or safflower oil, and wipe off the excess with a cloth until you can’t really see any on the surface and put your knife away. The oil will help block oxygen from getting to the iron in the blade so iron-oxide (rust) does not develop. (If you do happen to oil your blade with a non-drying oil or something else that would be bad for a painting like (like WD-40) usually warm soapy water can clean it. Just wash it really well with a dish-soap and dry it and then apply a good oil).
Knives that are polished to a reflective finish don’t rust as easily as those without, but some knives that have a reflective finish are not polished, but are rather plated with a metal like nickel. I prefer knives that are polished rather than plated because I know that if I were to ever have a knife that rusted or simply became too scratched from regular use I could simply polish it again with using my mad gunsmithing and armouring skills using my variable speed bench grinder on a soft wheel charged with green metal polish for a mirror finish or black polish for a satin finish. I also don’t like plated knives because plating changes the response of the blade. Because my standard knife is polished I don’t worry about rust very much and don’t usually have to oil my blade.
If you want a good piece of japanese steel either ask Hattori Hanzo to make one for you or you can go to dickblick.com and get a reasonably priced knife. I use the larger one and it seems to do the job very well. I’ve had mine for years and while I’m pretty rough on him he’s still cutting through paint like Uma cut through the Crazy 88.