Im not to big into complicated paint jobs etc. But sometimes I like to make my cars my own. Give em a little touch of what I want. But i’ve been having problems when it comes to FH3. For example my favorite lambo. In the paint booth and in photo mode it looks flawless. When you’re out driving tho there is a big white line that’s added on the left side under the window. Others can see it as well. Really annoying. Just tried to give the R-34 a custom paint job. Left side of the car is just fine. The right side something messes up the layer. I can upload a video on youtube if anyone want to see. But im starting to get demotivated to giving my own cars some personality, when things just keeps messing up. I know this is just two examples. But why bother when bugs up. Since im not good at this I tend to spend some time doing what im sure would be considered simple jobs. And to see that your plans and visions just goes bye bye takes away the drive to even continue. So im wondering. Have anyone else been having troubles when they try to create their own cars? Maybe it can be a thread where people report problems. So maybe (I doubt it tho) but maybe things will get patched up with time.
Welcome to the paint booth! 
Unfortunately, there are ‘issues’ you’ll need to work around when creating liveries in Forza… always have been & (most likely) always will be. I don’t know enough about it myself but I’ve often read that people think these issues are easily remedied. It’s frustrating at times but I also quite enjoy the challenge of trying to get over the glitches.
Having said that, I can’t honestly remember a more glitchy edition of a Forza game. I’ve had to compromise most of my designs in FH3 but in the end I figure I’m the only one who knows what I intended them to look like so nobody else is going to be too bothered - if they like it they’ll download it.
There have been threads dedicated to paint glitches in the past but I don’t think there’s one in FH3, you might want to do a search but I found a couple of older threads on the subject…
Lamborghini Huracan painting bug problem/complaint
Having problems with the vinyl changes once I’m able to drive my own car (this one in particular has the same issue with the Lambo).
[Mod Edit - Abbreviated profanity, profanity and profanity that is disguised but still alludes to the words are not permitted - D]