Plastics - flex agent & primer?

[AdamD]

I am preparing to paint a bike that has some plastics (fenders, etc.). I've heard before that a flex agent is commonly used for painting on plastics. When I asked the guy at the paint shop about it, he said it wouldn't be necessary with the supplies that I bought. I have ppg base coat black and some urethane clear coat. Can any one advise me on this? Any help would be greatly appreciated.

Also, would it be necessary for me to use primer on plastics that are already painted? I know I'll definitely need to spray some primer on the metal gas tank, which has had some minor body work. Again, I'm going black with the base coat, so I'm wondering if primer over the existing color, after sanding of course, would just be a waste of time. Would the black appear slightly different due to the color, in this case orange, underneath it compared to the black over gray primer on the tank /ubbthreads/images/graemlins/newbie.gif?

 

[TAZ]

I treat painted plastics like painted metal parts. Todays paints are very flexible so no need for flex agent. You could spray a test panel and put 10 coats on it...a month later you could bend it quite a bit and it wouldn't crack on you.

If you have spots that you go to the plastic, go ahead and use regular catalyzed primer. You won't have any problem. Just don't do like some newbies would and wetsand it with a very fine grit, just make sure it's roughed up good with about a 320 grit paper.

Now, if you have a 'raw plastic' piece made of ABS, you would need to pre this differently.

 

[AdamD]

Thanks a lot for the expert insight. Now it's just a matter of finding enough free time to get it done.
If it turns out good I'll post it. If not, I'm sure I'll have learned something and will prepare for a second attempt.
It seems the learning curve is increased in direct proportion to the pain of the mistakes.