Your choice. They are not coated with anything after they are printed. Generally, I do not coat mine with a liquid or aerosol decal film. If I have trouble with the first few I use, then I will coat them.
I recommend cutting the images from the sheet with a very sharp, hobby knife. Try not to cut too close to the image as it may affect them image. I recommend placing the image on a damp cloth or sponge until the image and decal film will slide easily from the backing paper. Move the paper and image to the model and slide the image from the paper into place. I DO NOT recommend soaking the decals in a bowl of water. This increases the risk of the image floating free, folding, or loosing colors. I do recommend applying a decal setting solution to the model before applying the image. It can make it easier to slide the decal into place.
The images are not individual decals. The decal film covers then entire sheet of paper. If the decal sheet is soaked in water, the entire set of decal images will lift from the decal paper.
I recommend the surface of the model or miniature have a smooth, clean, gloss surface.
I use decal solvents and setting solutions from Microscale Industries and I-94 Enterprises and have not experienced any problems with the decals. If you're concerned about a potential problem I recommend testing the product on an unneeded decal or the item description portion of the sheet.
I do not recommend using oil washes over these decals. In my experience, some of the graphics have come away from the surface when the oil-wash is being removed.
My primary printer is an Oki 942DN. I also use an Oki C711WT. Both printers will print white. The Oki 942 prints white first and then over-prints cyan, magenta, and yellow. It also print black. The Oki C711WT prints cyan, magenta, yellow, and then prints white last.