Delrin seems soft for a motor mount but it depends on your specific set-up. I've painted aluminum with pretty good luck with no corrosion with steel in a wet climate even. For next time tapping in aluminum, maybe try roll-form taps. I've had really good luck with them for smaller holes. They mash the aluminum into threads instead of cutting the metal. I'm sure someone on this site told me about them a while back. I use some aluminum tapping oil (green can) I got at a building supply store years ago that works well. Once I started with roll-forming taps on Al, I've only broken one 4-40 tap in hundreds of 4-40 - 1/4-20 tapped holes. I broke "many" of the regular cutting taps in Al. Be advised though that drill hole sizes need to be a bit different for those taps.