I'm sorry to hear about the issues you're having when attempting to print bumper stickers and stickers for car windows. Unfortunately, the print fading that you're experiencing is a limitation of inkjet printing in general. It's unlikely that it has anything to do with your specific printer or ink.
Most of the bumper stickers or car window stickers that are available on the market are printed using silk screen printers with UV resistant ink and a UV laminate. It's not an easy process to replicate at a reasonable budget. Most companies that offer custom printed labels are running a large press dedicated to screen printing and are capable of producing stickers at extremely high volumes. I'm unaware of a similar printing process that would be available at a scale that is suitable for smaller businesses or at-home printing.
The problem with inkjet printing for this type of application is that the ink itself won't have UV resistant capabilities and that it's ultimately left exposed on the surface of the labels. The inkjet topcoating featured on the weatherproof glossy label material does an excellent job of protecting against contact with moisture, but direct and consistent exposure to sunlight will cause the ink to fade over time. The effects of consistent UV exposure can be limited to some degree by using a UV protective spray like the
Krylon UV-Resistant Clear Coating, but this will likely just delay the rate at which the labels fade rather than make them fade-resistant permanently.
You may want to consider doing some testing with laser printing. Some of the same issues are still there - namely, that the toner from a laser printer will still be left exposed and unprotected from UV exposure when in direct exposure to sunlight. I do expect that toner will hold up considerably longer in direct sunlight than ink from a desktop inkjet printer. If you have access to a laser printer for testing purposes I'd recommend trying our weatherproof vinyl laser material. This material is typically meant for labeling chemical drums for maritime shipping applications, so it's one of the more durable options that we provide. You can view the full sheet configuration in this material type at the link below:
https://www.onlinelabels.com/products/OL3536LV.htm
If you're interested in trying this product you can request a few free sample sheets at the following link:
https://www.onlinelabels.com/general/SamplesSelectItems.aspx
The weatherproof vinyl laser material will probably offer the best solution for labels that will be exposed to consistent and direct sunlight, but it is important to note that they are not likely to provide a permanent solution to the problem. Fading will still likely occur after enough exposure, though I do expect that they'll last for a while longer than comparable inkjet products. To get a truly "fade-proof" bumper sticker label you'd almost certainly need to look into having them screen printed with a UV laminate, as mentioned above.
I hope this helps. If you need any additional information please just let me know.